Signal processing circuit and method for driving the same

ABSTRACT

An object is to provide a signal processing circuit which can be manufactured without a complex manufacturing process and suppress power consumption. A storage element includes two logic elements (referred to as a first phase-inversion element and a second phase-inversion element) which invert a phase of an input signal and output the signal, a first selection transistor, and a second selection transistor. In the storage element, two pairs each having a transistor in which a channel is formed in an oxide semiconductor layer and a capacitor (a pair of a first transistor and a first capacitor, and a pair of a second transistor and a second capacitor) are provided. The storage element is used in a storage device such as a register or a cache memory included in a signal processing circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/215,302, filed Aug. 23, 2011, now allowed, which claims the benefitof foreign priority applications filed in Japan as Serial No.2010-189214 on Aug. 26, 2010, and Serial No. 2011-113178 on May 20,2011, all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

An embodiment of the present invention relates to a nonvolatile storagedevice which can keep a stored logic state even when power is turned offand also relates to a signal processing circuit including thenonvolatile storage device. Further, an embodiment of the presentinvention relates to methods for driving the storage device and thesignal processing circuit. Furthermore, an embodiment of the presentinvention relates to an electronic device including the signalprocessing circuit.

2. Description of the Related Art

A signal processing circuit such as a central processing unit (CPU) hasa variety of configurations depending on its application but isgenerally provided with some kinds of storage devices such as a registerand a cache memory as well as a main memory for storing data or aprogram. A register has a function of temporarily holding data forcarrying out arithmetic processing, holding a program execution state,or the like. In addition, a cache memory is located between anarithmetic circuit and a main memory in order to reduce low-speed accessto the main memory and speed up the arithmetic processing.

In a storage device such as a register or a cache memory, writing ofdata needs to be performed at higher speed than in a main memory. Thus,in general, a flip-flop or the like is used as a register, and a staticrandom access memory (SRAM) or the like is used as a cache memory. Thatis, for such a register, a cache memory, or the like, a volatile storagedevice in which data is erased when supply of a power voltage isstopped.

In order to reduce consumed power, a method for temporarily stopping asupply of a power-supply voltage to a signal processing circuit in aperiod during which data is not input and output has been suggested. Inthe method, a nonvolatile storage device is located in the periphery ofa volatile storage device such as a register or a cache memory, so thatthe data is temporarily stored in the nonvolatile storage device. Thus,the register, the cache memory, or the like holds data even while asupply of power voltage is stopped in the signal processing circuit (forexample, see Patent Document 1).

In addition, in the case where a supply of the power-supply voltage isstopped for a long time in a signal processing circuit, data in avolatile storage device is transferred to an external storage devicesuch as a hard disk or a flash memory before the supply of thepower-supply voltage is stopped, so that the data can be prevented frombeing erased.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    H10-078836

SUMMARY OF THE INVENTION

In the case where data of a volatile storage device is stored in anonvolatile storage device located in the periphery of the volatilestorage device while the supply of power-supply voltage is stopped in asignal processing circuit, such a nonvolatile storage device is mainlyformed using a magnetic element or a ferroelectric; thus, themanufacturing process of the signal processing circuit is complex. Inaddition, in such a nonvolatile storage device, there is a limitation onthe number of data rewriting operations because of deterioration of astorage element due to a repeat of writing data and erasing data.

In the case where data of the volatile storage device is stored in theexternal storage device while a supply of power-supply voltage isstopped in the signal processing circuit, it takes a long time forreturning data from the external storage device to the volatile storagedevice. Therefore, back up of data using the external storage device isnot suitable in the case where the power supply is stopped for a shorttime so as to reduce consumed power.

In view of the above-described problems, it is an object of oneembodiment of the present invention to provide a signal processingcircuit whose consumed power can be suppressed and a method for drivingthe signal processing circuit. In particular, it is an object to providea signal processing circuit whose consumed power can be suppressed bystopping the power supply for a short time and a method for driving thesignal processing circuit.

An embodiment of the present invention is a storage element includingtwo logic elements (hereinafter, the logic elements are referred to asphase-inversion elements, i.e., a first phase-inversion element and asecond phase-inversion element) which invert a phase of an input signaland output the signal, a first selection transistor, and a secondselection transistor. In the storage element, two pairs each having atransistor in which a channel is formed in an oxide semiconductor layerand a capacitor (a pair of a first transistor and a first capacitor, anda pair of a second transistor and a second capacitor) are provided.

For the oxide semiconductor layer, for example, an In—Ga—Zn—O-basedoxide semiconductor material can be used.

A potential of an output terminal of a first phase-inversion element issupplied to an input terminal of a second phase-inversion element via afirst transistor which is on, and a potential of an output terminal of asecond phase-inversion element is supplied to an input terminal of thefirst phase-inversion element via a second transistor which is on. Oneof a pair of electrodes of the first capacitor is electrically connectedto the first transistor and the input terminal of the secondphase-inversion element. In other words, even when the first transistoris off, the first capacitor holds the potential of the input terminal ofthe second phase-inversion element. One of a pair of electrodes of thesecond capacitor is electrically connected to the second transistor andthe input terminal of the first phase-inversion element. In other words,even when the second transistor is off, the second capacitor holds thepotential of the input terminal of the first phase-inversion element.

A constant potential is supplied to the other electrode of the firstcapacitor and the other electrode of the second capacitor. For example,a reference potential (GND) is supplied.

A potential of a signal (data) input to the storage element is suppliedto the input terminal of the first phase-inversion element via the firstselection transistor and the second transistor which are on. A potentialof the output terminal of the first phase-inversion element is output asan output signal of the storage element via the second selectiontransistor which is on.

The first transistor and the second transistor are controlled so thatwhen one of the transistors is on, the other is also on. For example, inthe case where the first transistor and the second transistor have thesame conductivity, a first control signal input to a gate of the firsttransistor and a second control signal input to a gate of the secondtransistor are the same signal.

In the above storage element, in the case where in order to reduceconsumed power in data holding, after a supply of power-supply voltage,the supply of the power-supply voltage is stopped and then thepower-supply voltage is supplied again, a driving method can be asfollows.

First, the case where the power-supply voltage is supplied to thestorage element is described. That is, the case where the power-supplyvoltage is supplied to the first phase-inversion element and the secondphase-inversion element is described. The first selection transistor isturned on in the state where the first transistor and the secondtransistor are on. Thus, the input signal (data) is input to the inputterminal of the first phase-inversion element. Then, the first selectiontransistor is turned off, whereby the data is held by a feedback loopformed with the first phase-inversion element and the secondphase-inversion element. The potential of the input terminal of thesecond phase-inversion element is held by the first capacitor, and thepotential of the input terminal of the first phase-inversion element isheld by the second capacitor. Note that the second transistor is offwhile the data is being input and held. The second transistor is turnedon after the holding data is completed, whereby the data can be read outfrom the storage element.

The case where the supply of power-supply voltage to the storage elementis stopped after the data holding is completed is described. That is,the case where the supply of the power-supply voltage to the firstphase-inversion element and the second phase-inversion element isdescribed. Before the supply of the power-supply voltage is stopped, thefirst transistor and the second transistor are turned off. Here, thepotential of the input terminal of the second phase-inversion element isheld by the first capacitor, and the potential of the input terminal ofthe first phase-inversion element is held by the second capacitor.Therefore, even when the supply of the power-supply voltage to the firstphase-inversion element and the second phase-inversion element isstopped, data can be continuously held in the storage element. While thesupply of the power-supply voltage to the first phase-inversion elementand the second phase-inversion element is stopped, the first transistorand the second transistor are off. During the period where the supply ofthe power-supply voltage to the first phase-inversion element and thesecond phase-inversion element is stopped, an output signal cannot beoutput from the storage element, and another input signal (data) cannotbe input to nor held in the storage element.

Next, the case where the power-supply voltage is supplied to the storageelement again is described. After the power-supply voltage is suppliedto the first phase-inversion element and the second phase-inversionelement, the first transistor and the second transistor are turned on.Thus, the storage element is in a state where an output signal can beoutput and another input signal (data) can be held.

That is the driving method of the above storage element in the casewhere the supply of the power-supply voltage is stopped in order toreduce power consumed in data holding after the supply of thepower-supply voltage, and then the power-supply voltage is suppliedagain.

Note that as the first phase-inversion element and the secondphase-inversion element, for example, an inverter, a clocked inverter,or the like can be used.

The above storage element is used for a storage device included in thesignal processing circuit. The storage device can be formed with atleast one storage element. For example, the above storage element isused for a storage device such as a register or a cache memory includedin the signal processing circuit.

Further, the signal processing circuit may include some kinds of logiccircuits such as an arithmetic circuit which transmits/receives datato/from the storage device in addition to the storage device. Not onlythe supply of power-supply voltage to the storage device but also thesupply of power-supply voltage to the arithmetic circuit whichtransmits/receives data to/from the storage device may be stopped.

The storage device may have a switching element which controls thesupply of power-supply voltage to a storage element. In the case wherethe supply of power-supply voltage to the arithmetic circuit is stopped,the arithmetic circuit may include a switching element which controlsthe supply of power-supply voltage.

The off-state current of a transistor in which a channel is formed in anoxide semiconductor layer is extremely low. For example, the off-statecurrent of the transistor in which a channel is formed in an oxidesemiconductor layer is significantly lower than that of a transistor inwhich a channel is formed in silicon having crystallinity. Thus, whenthe transistor including an oxide semiconductor is used for the firsttransistor and the second transistor, potentials held in the firstcapacitor and the second capacitor are held while the power-supplyvoltage is not supplied to the storage element. The storage element canaccordingly hold the stored content while the supply of the power-supplyvoltage is stopped.

In the storage element, even while the supply of power-supply voltage isstopped, a potential of the input terminal of the second phase-inversionelement is held by the potential held in the first capacitor, and apotential of the input terminal of the first phase-inversion element isheld by the potential held in the second capacitor. That is, both thepotential of the input terminal of the first phase-inversion element andthe potential of the input terminal of the second phase-inversionelement are held.

On the other hand, for example, the case where the storage elementincludes the first capacitor and the first transistor but does notinclude the second capacitor and the second transistor is considered.That is, the case where the output terminal of the secondphase-inversion element is directly connected to the input terminal ofthe first phase-inversion element is considered. In such a structure,the potential of the input terminal of the second phase-inversionelement is held by the potential held in the first capacitor, but thepotential of the input terminal of the first phase-inversion element isnot held. Thus, by turning the first transistor on after a supply of thepower-supply voltage is resumed, electric charges transfer so that thepotential of the input terminal of the first phase-inversion element isset to a predetermined potential (a potential determined by an output ofthe second phase-inversion element). The storage element cannot outputdata until transfer of the electric charges is completed. Thus, a timeelapsing before the storage element can output data again (hereinafter,referred to as a rising time) is long. That is, it takes a long time forthe storage element to return to the state same as that before thesupply of the power is stopped.

In the storage element according to the present invention, while thesupply of the power-supply voltage is stopped, both the potential of theinput terminal of the first phase-inversion element and the potential ofthe input terminal of the second phase-inversion element are held. Thus,when the first transistor and the second transistor are turned on afterthe supply of the power-supply voltage to the storage element isresumed, the electrical charges do not need to transfer so that thepotential of the input terminal of the second phase-inversion elementand the potential of the input terminal of the first phase-inversionelement are to be the predetermined potential, and accordingly therising time can be short.

By applying such a storage element to a storage device such as aregister or a cache memory included in a signal processing circuit, datain the storage device can be prevented from being erased owing to thestop of the supply of the power-supply voltage. In addition, after thesupply of the power-supply voltage is resumed, the storage element canreturn to the state same as that before the power-supply voltage isstopped in a short time. Therefore, the power supply can be stopped evenfor a short time in the signal processing circuit or one or a pluralityof logic circuits included in the signal processing circuit.Accordingly, it is possible to provide a signal processing circuit whoseconsumed power can be suppressed and a method for driving the signalprocessing circuit whose consumed power can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are circuit diagrams of a storage element.

FIG. 2 is a timing chart showing the operation of a storage element.

FIGS. 3A and 3B each illustrate a structure of a storage device.

FIG. 4 is a block diagram of a signal processing circuit.

FIG. 5 is a block diagram of a CPU in which a storage device is used.

FIG. 6 is a cross-sectional view illustrating a structure of a storageelement.

FIG. 7 is a cross-sectional view illustrating a structure of a storageelement.

FIG. 8 is a cross-sectional view illustrating a structure of a storageelement.

FIG. 9 is a cross-sectional view illustrating a structure of a storageelement.

FIGS. 10A to 10E illustrate a method for manufacturing a storageelement.

FIGS. 11A to 11C illustrate the method for manufacturing a storageelement.

FIGS. 12A to 12D illustrate the method for manufacturing a storageelement.

FIGS. 13A to 13D illustrate the method for manufacturing a storageelement.

FIGS. 14A to 14D illustrate the method for manufacturing a storageelement.

FIGS. 15A to 15D illustrate the method for manufacturing a storageelement.

FIGS. 16A to 16C illustrate a method for manufacturing a storageelement.

FIGS. 17A and 17B are cross-sectional views each illustrating astructure of a transistor.

FIGS. 18A to 18F each illustrate a structure of an electronic device.

FIGS. 19A to 19E illustrate structures of oxide materials.

FIGS. 20A to 20C illustrate a structure of an oxide material.

FIGS. 21A to 21C illustrate a structure of an oxide material.

FIG. 22 shows dependence of mobility on gate voltage obtained bycalculation.

FIGS. 23A to 23C show gate voltage dependence of drain current andmobility obtained by calculation.

FIGS. 24A to 24C show gate voltage dependence of drain current andmobility obtained by calculation.

FIGS. 25A to 25C show gate voltage dependence of drain current andmobility obtained by calculation.

FIGS. 26A and 26B illustrate cross-sectional structures of transistorsused for calculation.

FIGS. 27A to 27C are graphs showing characteristics of transistors eachincluding an oxide semiconductor film.

FIGS. 28A and 28B are graphs showing V_(g)−I_(d) characteristics after aBT test of a transistor of Sample 1.

FIGS. 29A and 29B are graphs showing V_(g)−I_(d) characteristics after aBT test of a transistor of Sample 2.

FIG. 30 shows XRD spectra of Sample A and Sample B.

FIG. 31 is a graph showing a relation between the off-state current andthe substrate temperature in measurement of a transistor.

FIG. 32 is a graph showing V_(g) dependence of I_(d) and field-effectmobility.

FIG. 33A is a graph showing a relation between the threshold voltage andthe substrate temperature, and FIG. 33B is a graph showing a relationbetween the field-effect mobility and the substrate temperature.

FIGS. 34A and 34B are a top view and a cross-sectional view of atransistor.

FIGS. 35A and 35B are a top view and a cross-sectional view of atransistor.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the following description and it is easilyunderstood by those skilled in the art that the mode and details can bevariously changed without departing from the scope and spirit of thepresent invention. Accordingly, the invention should not be construed asbeing limited to the description of the embodiments below.

Note that functions of the “source” and “drain” may be switched in thecase where transistors of different polarities are employed or in thecase where the direction of a current flow changes in a circuitoperation, for example. Therefore, the terms “source” and “drain” can beused to denote the drain and the source, respectively, in thisspecification.

Note that in this specification and the like, the term “electricallyconnected” includes the case where components are connected through an“object having any electric function”. There is no particular limitationon an object having any electric function as long as electric signalscan be transmitted and received between components that are connectedthrough the object. Examples of an “object having any electric function”are a switching element such as a transistor, a resistor, an inductor, acapacitor, and an element with a variety of functions as well as anelectrode and a wiring.

In addition, even when a circuit diagram shows independent components asif they are electrically connected to each other, there is actually acase where one conductive film has functions of a plurality ofcomponents such as a case where part of a wiring also functions as anelectrode. The “electrical connection” in this specification includes inits category such a case where one conductive film has functions of aplurality of components.

In this specification and the like, the terms “over” and “below” do notnecessarily mean “directly on” and “directly below”, respectively, inthe description of a physical relationship between components. Forexample, the expression “a gate electrode over a gate insulating layer”can mean the case where there is an additional component between thegate insulating layer and the gate electrode.

Note that the position, size, range, or the like of each componentillustrated in drawings and the like is not accurately represented insome cases for easy understanding. Therefore, the disclosed invention isnot necessarily limited to the position, size, range, or the like asdisclosed in the drawings and the like.

The ordinal number such as “first”, “second”, and “third” are used inorder to avoid confusion among components.

Embodiment 1

A signal processing circuit includes a storage device. The storagedevice includes one or a plurality of storage elements which can store1-bit data.

Note that a CPU, a large scale integrated circuit (LSI) such as amicroprocessor, an image processing circuit, a digital signal processor(DSP), or a field programmable gate array (FPGA), and the like areincluded in the signal processing circuit of the preset invention in itscategory.

FIG. 1A illustrates an example of a circuit diagram of a storageelement. A storage element 100 illustrated in FIGS. 1A and 1B includes aphase-inversion element 101 and a phase-inversion element 102 each ofwhich inverts a phase of an input signal and outputs the signal, aselection transistor 103, a selection transistor 104, a transistor 111,a transistor 112, a capacitor 121, and a capacitor 122. In each of thetransistor 111 and the transistor 112, a channel is formed in an oxidesemiconductor layer. Note that the storage element 100 may furtherinclude another circuit element such as a diode, a resistor, or aninductor, as needed. In the circuit diagram of FIG. 1A, “OS” is writtenbeside a transistor in order to indicate that the transistor 111 and thetransistor 112 have a structure in which a channel is formed in an oxidesemiconductor layer.

A signal IN including data input to the storage element 100 is suppliedto an input terminal of the phase-inversion element 101 via theselection transistor 103 and the transistor 111 which are on. Further,the potential of an output terminal of the phase-inversion element 101is output as an output signal OUT of the storage element via theselection transistor 104 which is on. The signal OUT is output to astorage element of a subsequent stage or another circuit.

The potential of the output terminal of the phase-inversion element 101is supplied to an input terminal of the phase-inversion element 102 viathe transistor 112 which is on, and the potential of an output terminalof the phase-inversion element 102 is supplied to the input terminal ofthe phase-inversion element 101 via the transistor 111 which is on. Oneof a pair of electrodes of the capacitor 122 is electrically connectedto the transistor 112 and the input terminal of the phase-inversionelement 102. In other words, even when the transistor 112 is off, thecapacitor 122 holds the potential of the input terminal of thephase-inversion element 102. One of a pair of electrodes of thecapacitor 121 is electrically connected to the transistor 111 and theinput terminal of the phase-inversion element 101. In other words, evenwhen the transistor 111 is off, the capacitor 121 holds the potential ofthe input terminal of the phase-inversion element 101.

Note that the other electrode of the capacitor 121 and the otherelectrode of the capacitor 122 are supplied with the constant potentialVSS. For example, the potential VSS can be the reference potential(GND).

A control signal S1 is input to a gate of the transistor 111, a controlsignal S2 is input to a gate of the transistor 112, a control signal S3is input to a gate of the selection transistor 103, and a control signalS4 is input to a gate of the selection transistor 104. For example, inthe case where the transistor 111 and the transistor 112 have the sameconductivity, the control signal S1 and the control signal S2 can be thesame signal. That is, the transistor 111 and the transistor 112 arecontrolled so that when one of them is on, the other is also on.

Note that FIG. 1A illustrates an example in which inverters are used asthe phase-inversion element 101 and the phase-inversion element 102.However, any of elements may be employed as the phase-inversion element101 and the phase-inversion element 102 as long as the element inverts aphase of an input signal and outputs the signal. A clocked inverter orthe like can be used.

FIG. 1B illustrates an example in which inverters including an n-channeltransistor and a p-channel transistor are used as the phase-inversionelement 101 and the phase-inversion element 102 of FIG. 1A. Thephase-inversion element 101 includes an n-channel transistor 131 and ap-channel transistor 132, and the phase-inversion element 102 includesan n-channel transistor 133 and a p-channel transistor 134.

A potential V1 is supplied to one of a source and a drain of then-channel transistor 131, and a potential V2 is supplied to one of asource and a drain of the p-channel transistor 132. The other of thesource and the drain of the n-channel transistor 131 and the other ofthe source and the drain of the p-channel transistor 132 areelectrically connected to each other. A gate of the n-channel transistor131 and a gate of the p-channel transistor 132 are the input terminal ofthe phase-inversion element 101. The other of the source and the drainof the n-channel transistor 131 and the other of the source and thedrain of the p-channel transistor 132 are the output terminal of thephase-inversion element 101.

The potential V1 is supplied to one of a source and a drain of then-channel transistor 133, and the potential V2 is supplied to one of asource and a drain of the p-channel transistor 134. The other of thesource and the drain of the n-channel transistor 133 and the other ofthe source and the drain of the p-channel transistor 134 areelectrically connected to each other. A gate of the n-channel transistor133 and a gate of the p-channel transistor 134 are the input terminal ofthe phase-inversion element 102. The other of the source and the drainof the re-channel transistor 133 and the other of the source and thedrain of the p-channel transistor 134 are the output terminal of thephase-inversion element 102.

In the phase-inversion element 101 and the phase-inversion element 102,when the power-supply voltage is supplied, the potential V2 is higherthan the potential V1. The difference between the potential V1 and thepotential V2 is the power-supply voltage of the phase-inversion element101 and the phase-inversion element 102. For example, in thephase-inversion element 101 and the phase-inversion element 102, whenthe power-supply voltage is supplied, the potential V2 can be thepotential VDD, and the potential V1 can be the potential VSS. Further,the potential VSS can be the reference potential (GND). On the otherhand, in the phase-inversion element 101 and the phase-inversion element102, when the supply of the power-supply voltage is stopped, the stop ofsupply corresponds to the case where supply of one of the potential V1and the potential V2 or the both is stopped, for example. Alternatively,the stop of supply corresponds to the case where both the potential V1and the potential V2 are the reference potential (GND), for example.

Further, in one embodiment of the present invention, at least thetransistor 111 and the transistor 112 are transistors in which a channelis formed in an oxide semiconductor layer. Thus, the selectiontransistor 103, the selection transistor 104, the transistors used inthe phase-inversion element 101 and the phase-inversion element 102 canbe transistors in which a channel is formed in a semiconductor layer ora semiconductor substrate including a semiconductor other than an oxidesemiconductor. For the oxide semiconductor layer, for example, anIn—Ga—Zn—O-based oxide semiconductor material can be used. Asemiconductor other than an oxide semiconductor can be an amorphoussemiconductor, a microcrystalline semiconductor, a polycrystallinesemiconductor, or a single crystal semiconductor. Silicon or germaniumcan be used.

The transistors used as the transistor 111 and the transistor 112 can betransistors in which a channel is formed in a highly purified oxidesemiconductor layer. The off-state current density of such a transistorcan be less than or equal to 100 zA/μm, preferably less than or equal to10 zA/μm, further preferably less than or equal to 1 zA/μm. Thus, theoff-state current of the transistor is extremely lower than that of thetransistor including silicon with crystallinity. As a result, when thetransistor 111 and the transistor 112 are off, the electric chargesstored in the capacitor 121 and the capacitor 122 are hardly discharged,and thus the data of the storage element 100 can be held.

A material which can realize the off-state current characteristicsequivalent to those of the oxide semiconductor material, such as a widegap material like silicon carbide (more specifically, a semiconductormaterial with an energy gap Eg of greater than 3 eV) may be used insteadof the oxide semiconductor material.

Next, an example of the operation of the storage element 100 illustratedin FIGS. 1A and 1B is described. Is described the operation of thestorage element 100 in the case where after the power-supply voltage issupplied, the supply of the power supply voltage is stopped in order toreduce power consumed in data holding and then the power-supply voltageis supplied again, with reference to a timing chart of FIG. 2.

Note that the timing chart shows, as an example, the case where all ofthe selection transistor 103, the selection transistor 104, thetransistor 111, and the transistor 112 are re-channel transistors. Inaddition, an example in which the transistors are turned on when ahigh-level potential is input to the gates, and the transistors areturned off when the low-level potential is input to the gates. However,the operation of the storage element is not limited to the above. Theselection transistor 103, the selection transistor 104, the transistor111, and the transistor 112 may be n-channel transistors or p-channeltransistors. A potential of each signal may be determined so that statesof the transistors (the on state or the off state) are similar to thosein the following description.

An example in which the case where the signal IN is at high levelcorresponds to data “1” and the case where the signal IN is at low levelcorresponds to data “0” is shown; however, the data is not limited tothe above. The case where the signal IN is at low level may correspondto data “1”, and the case where the signal IN is at high level maycorrespond to data “0”.

First, the case where a power-supply voltage (indicated as V in FIG. 2)is supplied to the storage element 100 is described. That is, the casewhere the power-supply voltage is supplied to the phase-inversionelement 101 and the phase-inversion element 102 is described. This casecorresponds to a period 1 in FIG. 2. The control signal S1 and thecontrol signal S2 are set to a high level, and the transistor 111 andthe transistor 112 are on. In that state, the control signal S3 is setto a high level, whereby the selection transistor 103 is turned on.Thus, the signal IN is input to the input terminal of thephase-inversion element 101. The signal IN has a potential correspondingto data stored while the selection transistor 103 is on (that is, whilethe control signal S3 is at high level). Here, for example, thepotential is a high-level potential corresponding to data “1”. Such ahigh-level potential is input to the input terminal of thephase-inversion element 101. Then, the control signal S3 is set to a lowlevel, and the selection transistor 103 is turned off, whereby the inputdata is held by a feedback loop formed with the phase-inversion element101 and the phase-inversion element 102. The potential of the inputterminal of the phase-inversion element 102 is held in the capacitor122, and the potential of the input terminal of the phase-inversionelement 101 is held in the capacitor 121. Note that while the data isbeing input and held, the control signal S4 is at low level, and theselection transistor 104 is off. After the holding data is completed,the control signal S4 is set to a high level, and the selectiontransistor 104 is turned on, whereby the signal OUT is output. The dataheld by the phase-inversion element 101 and the phase-inversion element102 is reflected to the signal OUT. Therefore, by reading the potentialof the signal OUT, the data can be read out from the storage element100. In the period 1 of the timing chart in FIG. 2, the data “1” is heldby the phase-inversion element 101 and the phase-inversion element 102;thus, while the control signal S4 is at high level and the selectiontransistor 104 is on, the signal OUT is at low level.

Next, the case where after the holding data is completed, the supply ofthe power-supply voltage to the storage element 100 is stopped in orderto reduce power consumed in the data holding is described. That is, thecase where the supply of the power-supply voltage to the phase-inversionelement 101 and the phase-inversion element 102 is stopped is described.This case corresponds to a period 2 in FIG. 2. Before the supply of thepower-supply voltage is stopped, the control signal S1 and the controlsignal S2 are set to a low level, and the transistor 111 and thetransistor 112 are turned off (see an instant before the period 2 ofFIG. 2). Since the off-state currents of the transistor 111 and thetransistor 112 are extremely low, the potential of the input terminal ofthe phase-inversion element 102 is held in the capacitor 122, and thepotential of the input terminal of the phase-inversion element 101 isheld in the capacitor 121. Thus, even when the supply of thepower-supply voltage to the phase-inversion element 101 and thephase-inversion element 102 is stopped, the storage element 100 cancontinuously hold data. While the supply of the power-supply voltage tothe phase-inversion element 101 and the phase-inversion element 102 isstopped, the control signal S1 and the control signal S2 are at lowlevel, and the transistor 111 and the transistor 112 are off. While thesupply of the power-supply voltage to the phase-inversion element 101and the phase-inversion element 102 is stopped, the signal OUT cannot beoutput from the storage element 100, and another signal IN cannot beinput to nor held in the storage element 100.

Note that when the supply of the power-supply voltage to the storageelement 100 is stopped, the transistor 111 and the transistor 112 shouldbe turned off before the supply of the power-supply voltage to thestorage element 100 is stopped. If the transistor 111 and the transistor112 were turned off after the supply of the power-supply voltage to thestorage element 100 is stopped, the following problem occurs. By thestop of the supply of the power-supply voltage to the storage element100, data cannot be held by the feedback loop formed with thephase-inversion element 101 and the phase-inversion element 102. Thus,when the transistor 111 and the transistor 112 are turned off after thesupply of the power-supply voltage to the storage element 100 isstopped, data cannot be held in the capacitor 121 and the capacitor 122.Therefore, in the case where the supply of the power-supply voltage tothe storage element 100 is stopped, the transistor 111 and thetransistor 112 should be turned off before the supply of thepower-supply voltage to the storage element 100 is stopped.

Note that FIG. 2 shows the example in which operation of the period 2 isperformed after data is held in the storage element 100 and the data isread out in the period 1 is shown; however, the operation is not limitedthereto. Data is held in the storage element 100 in the period 1, andthe operation of the period 2 is performed before the data is read out.

Next, the case where the power-supply voltage is supplied to the storageelement again is described. This case corresponds to a period 3 in FIG.2. After the power-supply voltage is supplied to the phase-inversionelement 101 and the phase-inversion element 102, the control signal S1and the control signal S2 are set to a high level, and the transistor111 and the transistor 112 are turned on. The potential of the inputterminal of the phase-inversion element 102 is held in the capacitor122, and the potential of the input terminal of the phase-inversionelement 101 is held in the capacitor 121; thus, a state same as thatbefore the supply of the power-supply voltage to the storage element isstopped can be provided. Here, by setting the control signal S4 to ahigh level, the signal OUT is at low level. In such a manner, even whenthe supply of the power-supply voltage to the storage element isstopped, data can be held. After that, by operation similar to theoperation in the period 1, data can be input, held, and output.

Note that in the case where the supply of the power-supply voltage tothe storage element 100 is resumed, the transistor 111 and thetransistor 112 should be turned on after the supply of the power-supplyvoltage to the storage element 100 is resumed. If the transistor 111 andthe transistor 112 were turned on before the supply of the power-supplyvoltage to the storage element 100 is resumed, the following problemoccurs. Since the supply of the power-supply voltage to the storageelement 100 is not resumed even if the transistor 111 and the transistor112 were turned on, data cannot be held by the feedback loop formed withthe phase-inversion element 101 and the phase-inversion element 102.Thus, in the case where the supply of the power-supply voltage to thestorage element 100 is resumed, the transistor 111 and the transistor112 should be turned on after the supply of the power-supply voltage tothe storage element 100 is resumed.

The above is the driving method of the storage element 100 in the casewhere the power-supply voltage is supplied, the supply of thepower-supply voltage is stopped, and the power-supply voltage issupplied again.

The off-state current of the transistor in which a channel is formed inan oxide semiconductor layer is extremely low. For example, theoff-state current of the transistor in which a channel is formed in anoxide semiconductor layer is significantly lower than that of atransistor in which a channel is formed in silicon having crystallinity.Thus, when such a transistor including an oxide semiconductor is usedfor the first transistor 111 and the second transistor 112, potentialsheld in the capacitor 121 and the capacitor 122 are held while thepower-supply voltage is not supplied to the storage element 100. Thestorage element 100 can accordingly hold the stored content while thesupply of the power-supply voltage is stopped.

In the storage element 100, even while the supply of the power-supplyvoltage is stopped, the potential of the input terminal of thephase-inversion element 102 is held by the potential held in thecapacitor 122, and the potential of the input terminal of thephase-inversion element 101 is held by the potential held in thecapacitor 121. That is, both the potential of the input terminal of thephase-inversion element 101 and the potential of the input terminal ofthe phase-inversion element 102 are held.

On the other hand, for example, the case where the storage element 100includes the capacitor 122 and the transistor 112 but does not includethe capacitor 121 and the transistor 111 is considered. That is, thecase where the output terminal of the phase-inversion element 102 isdirectly connected to the input terminal of the phase-inversion element101 is considered. In such a structure, the potential of the inputterminal of the phase-inversion element 102 is held by the potentialheld in the capacitor 122, but the potential of the input terminal ofthe phase-inversion element 101 is not held. Thus, by turning thetransistor 112 on after the supply of the power-supply voltage to thestorage element 100 is resumed, electric charges transfer so that thepotential of the input terminal of the phase-inversion element 101 isset to a predetermined potential (a potential determined by an output ofthe phase-inversion element 102). The storage element 100 cannot outputdata until transfer of the electric charges is completed. Thus, a timeelapsing before the storage element 100 can output data again(hereinafter, referred to as a rising time) is long. That is, it takes along time to for the storage element to return to the state same as thatbefore the supply of the power is stopped.

With the structures illustrated in FIGS. 1A and 1B, the storage element100 holds both the potential of the input terminal of thephase-inversion element 101 and the potential of the input terminal ofthe phase-inversion element 102 even after the supply of thepower-supply voltage is stopped. Thus, when the transistor 111 and thetransistor 112 are turned on after the supply of the power-supplyvoltage to the storage element 100 is resumed, electric charges do notneed to transfer so that the potential of the input terminal of thephase-inversion element 102 and the potential of the input terminal ofthe phase-inversion element 101 are to be the predetermined potential,and accordingly the rising time can be short.

This embodiment can be combined as appropriate with any of the otherembodiments.

Embodiment 2

In this embodiment, a structure of a storage device including aplurality of storage elements described in Embodiment 1 will bedescribed.

FIG. 3A illustrates a structural example of a storage device of thisembodiment. The storage device illustrated in FIG. 3A includes aswitching element 401 and a storage element group 403 including aplurality of storage elements 402. Specifically, as each of the storageelements 402, the storage element 100 whose structure is described inEmbodiment 1 can be used. Each of the storage elements 402 included inthe storage element group 403 is supplied with the high-level powersupply potential VDD via the switching element 401. Further, each of thestorage elements 402 included in the storage element group 403 issupplied with a potential of the signal IN and the low-level powersupply potential VSS.

In FIG. 3A, a transistor is used for the switching element 401, and theswitching of the transistor is controlled by a control signal Sig Asupplied to a gate electrode thereof.

Note that in FIG. 3A, a structure in which the switching element 401includes only one transistor is illustrated; however, the presentinvention is not limited to this structure. In one embodiment of thepresent invention, the switching element 401 may include a plurality oftransistors. In the case where the plurality of transistors which serveas switching elements are included in the switching element 401, theplurality of transistors may be electrically connected to each other inparallel, in series, or in combination of parallel connection and seriesconnection.

Although the switching element 401 controls the supply of the high-levelpower supply potential VDD to each of the storage elements 402 includedin the storage element group 403 in FIG. 3A, the switching element 401may control the supply of the low-level power supply potential VSS. InFIG. 3B, an example of a storage device in which each of the storageelements 402 included in the storage element group 403 is supplied withthe low-level power supply potential VSS via the switching element 401is illustrated. The supply of the low-level power supply potential VSSto each of the storage elements 402 included in the storage elementgroup 403 can be controlled by the switching element 401.

This embodiment can be combined as appropriate with any of the otherembodiments.

Embodiment 3

In this embodiment, a structure of a signal processing circuit includingthe storage device described in Embodiment 2 or the storage elementdescribed in Embodiment 1 will be described.

FIG. 4 illustrates an example of a signal processing circuit accordingto an embodiment of the present invention. The signal processing circuitat least includes one or a plurality of arithmetic circuits and one or aplurality of storage devices. Specifically, a signal processing circuit150 illustrated in FIG. 4 includes an arithmetic circuit 151, anarithmetic circuit 152, a storage device 153, a storage device 154, astorage device 155, a control device 156, and a power supply controlcircuit 157.

The arithmetic circuits 151 and 152 each include, as well as a logiccircuit which carries out simple logic arithmetic processing, an adder,a multiplier, and various arithmetic circuits. The storage device 153functions as a register for temporarily holding data when the arithmeticprocessing is carried out in the arithmetic circuit 151. The storagedevice 154 functions as a register for temporarily holding data when thearithmetic processing is carried out in the arithmetic circuit 152.

In addition, the storage device 155 can be used as a main memory and canstore a program executed by the control device 156 as data or can storedata from the arithmetic circuit 151 and the arithmetic circuit 152.

The control device 156 is a circuit which collectively controlsoperations of the arithmetic circuit 151, the arithmetic circuit 152,the storage device 153, the storage device 154, and the storage device155 included in the signal processing circuit 150. Note that in FIG. 4,a structure in which the control device 156 is provided in the signalprocessing circuit 150 as a part thereof is illustrated, but the controldevice 156 may be provided outside the signal processing circuit 150.

By using the storage element described in Embodiment 1 or the storagedevice described in Embodiment 2 for the storage device 153, the storagedevice 154, and the storage device 155, data can be held even when thesupply of power-supply voltage to the storage device 153, the storagedevice 154, and the storage device 155 is stopped. In the above manner,the supply of the power-supply voltage to the entire signal processingcircuit 150 can be stopped, whereby power consumption can be suppressed.Alternatively, the supply of the power-supply voltage to one or aplurality of the storage device 153, the storage device 154, and thestorage device 155 can be stopped, whereby power consumed by the signalprocessing circuit 150 can be suppressed. After the supply of thepower-supply voltage is resumed, a state same as that before the supplyof power is stopped can be provided for a short time.

In addition, as well as stop of the supply of the power-supply voltageto the storage device, the supply of the power-supply voltage to thecontrol circuit or the arithmetic circuit which transmits/receives datato/from the storage device may be stopped. For example, when thearithmetic circuit 151 and the storage device 153 do not operate, thesupply of the power-supply voltage to the arithmetic circuit 151 and thestorage device 153 may be stopped.

In addition, the power supply control circuit 157 controls the level ofthe power-supply voltage which is supplied to the arithmetic circuit151, the arithmetic circuit 152, the storage device 153, the storagedevice 154, the storage device 155, and the control device 156 includedin the signal processing circuit 150. Further, in the case where thesupply of the power-supply voltage is stopped, a switching element forstopping the supply of the power-supply voltage may be provided for thepower supply control circuit 157, or for each of the arithmetic circuit151, the arithmetic circuit 152, the storage device 153, the storagedevice 154, the storage device 155, and the control device 156. In thelatter case, the power supply control circuit 157 is not necessarilyprovided in the signal processing circuit according to the presentinvention.

A storage device which functions as a cache memory may be providedbetween the storage device 155 that is a main memory and each of thearithmetic circuit 151, the arithmetic circuit 152, and the controldevice 156. By providing the cache memory, low-speed access to the mainmemory can be reduced and the speed of the signal processing such asarithmetic processing can be higher. By applying the above-describedstorage element also to the storage device functioning as a cachememory, power consumption of the signal processing circuit 150 can besuppressed. Further, after the supply of the power-supply voltage isresumed, a state same as that before the supply of power is stopped canbe provided for a short time.

This embodiment can be combined as appropriate with any of the otherembodiments.

Embodiment 4

In this embodiment, a configuration of a CPU, which is one of signalprocessing circuits according to one embodiment of the presentinvention, will be described.

FIG. 5 illustrates a configuration of the CPU in this embodiment. TheCPU illustrated in FIG. 5 mainly includes an arithmetic logic unit (ALU)901, an ALU controller 902, an instruction decoder 903, an interruptcontroller 904, a timing controller 905, a register 906, a registercontroller 907, a bus interface (Bus I/F) 908, a rewritable ROM 909, anda ROM interface (ROM I/F) 920, over a substrate 900. Further, the ROM909 and the ROM I/F 920 may be provided over different chips. Naturally,the CPU illustrated in FIG. 5 is only an example with a simplifiedconfiguration, and an actual CPU may employ a variety of configurationsdepending on the application.

An instruction which is input to the CPU through the Bus I/F 908 isinput to the instruction decoder 903 and decoded therein, and then,input to the ALU controller 902, the interrupt controller 904, theregister controller 907, and the timing controller 905.

The ALU controller 902, the interrupt controller 904, the registercontroller 907, and the timing controller 905 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 902 generates signals for controlling the drive of the ALU901. While the CPU is executing a program, the interrupt controller 904judges an interrupt request from an external input/output device or aperipheral circuit on the basis of its priority or a mask state, andprocesses the request. The register controller 907 generates an addressof the register 906, and reads/writes data from/to the register 906 inaccordance with the state of the CPU.

The timing controller 905 generates signals for controlling a drivetiming of the ALU 901, the ALU controller 902, the instruction decoder903, the interrupt controller 904, and the register controller 907. Forexample, the timing controller 905 is provided with an internal clockgenerator for generating an internal clock signal CLK2 on the basis of areference clock signal CLK1, and supplies the clock signal CLK2 to theabove circuits.

In the CPU of this embodiment, a storage element having the structuredescribed in any of the above embodiments is provided in the register906. The register controller 907 judges whether data is held by thefeedback loop of the phase-inversion element (which corresponds to acase where the transistor 111 and the transistor 112 are on) or data isheld in the capacitor (which corresponds to a case where the transistor111 and the transistor 112 are off) in the storage element in theregister 906. When holding data by the feedback loop of thephase-inversion element is selected, a power-supply voltage is suppliedto the storage element in the register 906. When holding data in thecapacitor is selected, the supply of the power-supply voltage to thestorage element in the register 906 can be stopped. The power supply canbe stopped by providing a switching element between a storage elementgroup and a node to which the power supply potential VDD or the powersupply potential VSS is supplied, as illustrated in FIG. 3A or FIG. 3B.

In such a manner, even in the case where the operation of the CPU istemporally stopped and the supply of the power-supply voltage isstopped, data can be held and power consumption can be reduced.Specifically, for example, while a user of a personal computer does notinput data to an input device such as a keyboard, the operation of theCPU can be stopped, so that the power consumption can be reduced.

Although the example of the CPU is described in this embodiment, thesignal processing circuit of the present invention is not limited to theCPU and can be applied to an LSI such as a microprocessor, an imageprocessing circuit, a digital signal processor (DSP), or a fieldprogrammable gate array (FPGA).

This embodiment can be combined as appropriate with any of the otherembodiments.

Embodiment 5

In this embodiment, a structure of a transistor or the like included ina signal processing circuit will be described with reference to FIG. 6,FIG. 7, FIG. 8, FIG. 9, and FIGS. 17A and 17B.

FIG. 6 illustrates an example of a structure of the storage element 100illustrated in the circuit diagram of FIG. 1A or FIG. 1B. FIG. 6 is across-sectional view of two transistors (a transistor 660 and atransistor 662) and a capacitor 664 which are components included in thestorage element 100. The transistor 662 is a transistor in which achannel is formed in an oxide semiconductor layer. The transistor 662can correspond to the transistor 111 or the transistor 112 in FIGS. 1Aand 1B. The transistor 660 is a transistor in which a channel is formedin a semiconductor (e.g., silicon or the like) other than the oxidesemiconductor. The transistor 660 can correspond to the selectiontransistor 103, the selection transistor 104, or a transistor includedin the phase-inversion element (the n-channel transistor 131, thep-channel transistor 132, the n-channel transistor 133, or the p-channeltransistor 134 in FIG. 1B). The capacitor 664 can correspond to thecapacitor 121 or the capacitor 122 in FIGS. 1A and 1B.

In the example of the structure illustrated in FIG. 6, one of a sourceand a drain of the transistor 660 is connected to one of a source andthe drain of the transistor 662, and the other of the source and thedrain of the transistor 662 is connected to one of a pair of electrodesof the capacitor 664. As an example of such a structure, the case wherethe transistor 660, the transistor 662, and the capacitor 664 correspondto the selection transistor 103, the transistor 111, and the capacitor121 in FIGS. 1A and 1B, respectively, is described.

Although both the transistor 660 and the transistor 662 are n-channeltransistors here, it is needless to say that p-channel transistors canbe used.

The transistor 660 illustrated in FIG. 6 includes a channel formationregion 616 provided over a substrate 600 including a semiconductormaterial (e.g., silicon), impurity regions 620 a and 620 b between whichthe channel formation region 616 is sandwiched, metal compound regions624 a and 624 b in contact with the impurity regions 620 a and 620 b, agate insulating layer 608 provided over the channel formation region616, and a gate electrode 610 provided over the gate insulating layer608. In addition, an element separation insulating layer 606 is providedover the substrate 600.

Note that a transistor whose source electrode and drain electrode arenot explicitly illustrated in a drawing may be referred to as atransistor for the sake of convenience. Further, in such a case, indescription of a connection of a transistor, a source region and asource electrode are collectively referred to as a “source electrode,”and a drain region and a drain electrode are collectively referred to asa “drain electrode”. In other words, in this specification, the term“source electrode” may include a source region and the term “drainelectrode” may include a drain region.

Note that in this specification, the impurity region 620 a and theimpurity region 620 b are collectively referred to as impurity regions620 in some cases. Further, in this specification, the metal compoundregion 624 a and the metal compound region 624 b are collectivelyreferred to as metal compound regions 624 in some cases.

An insulating layer 628 is provided over the transistor 660. For highintegration, as illustrated in FIG. 6, it is preferable that thetransistor 660 do not include a sidewall insulating layer. On the otherhand, in the case where the characteristics of the transistor 660 havepriority, sidewall insulating layers may be provided on side surfaces ofa gate electrode 610, and the impurity regions 620 including a pluralityof regions with different impurity concentrations may be provided. Here,the insulating layer 628 preferably has a surface with favorableflatness; for example, the surface of the insulating layer 628preferably has a root-mean-square (RMS) roughness of 1 nm or less. Inthis manner, a channel formation region (an oxide semiconductor layer644) of the transistor 662 is provided in an extremely flat regionhaving a root-mean-square (RMS) roughness of 1 nm or less, whereby thetransistor 662 which can prevent a malfunction such as a short-channeleffect and has favorable characteristics can be provided even when thetransistor 662 is miniaturized.

The transistor 662 in FIG. 6 includes the oxide semiconductor layer 644formed over the insulating layer 628, an electrode 642 a and anelectrode 642 b which are partly in contact with the oxide semiconductorlayer 644, a gate insulating layer 646 covering the oxide semiconductorlayer 644 and the electrodes 642 a and 642 b, and a gate electrode 648provided over the gate insulating layer 646 to overlap with the oxidesemiconductor layer 644. The electrode 642 a is connected to the metalcompound region 624 b of the transistor 660 with an electrode 503 formedin an opening portion provided in the insulating layer 628.

Note that in this specification, the electrode 642 a and the electrode642 b are collectively referred to as an electrode 642.

Here, it is preferable that the oxide semiconductor layer 644 be ahighly purified oxide semiconductor layer by sufficiently removingimpurities such as hydrogen or sufficiently supplying oxygen.Specifically, the concentration of hydrogen in the oxide semiconductorlayer 644 is lower than or equal to 5×10¹⁹ atoms/cm³, preferably lowerthan or equal to 5×10¹⁸ atoms/cm³, further preferably lower than orequal to 5×10¹⁷ atoms/cm³, for example. In addition, the concentrationof an alkali metal element in the oxide semiconductor layer 644 ispreferably reduced. For example, the concentration of sodium (Na) may belower than or equal to 5×10¹⁶ atoms/cm³, preferably lower than or equalto 1×10¹⁶ atoms/cm³, further preferably lower than or equal to 1×10¹⁵atoms/cm³; the concentration of lithium (Li) may be lower than or equalto 5×10¹⁵ atoms/cm³, preferably lower than or equal to 1×10¹⁵ atoms/cm³;and the concentration of potassium (K) may be lower than or equal to5×10¹⁵ atoms/cm³, preferably lower than or equal to 1×10¹⁵ atoms/cm³.

Note that it has been pointed out that an oxide semiconductor isinsensitive to impurities, there is no problem when a considerableamount of metal impurities is contained in the film, and therefore,soda-lime glass which is inexpensive can also be used (Kamiya, Nomura,and Hosono, “Carrier Transport Properties and Electronic Structures ofAmorphous Oxide Semiconductors The present status”, KOTAI BUTSURI (SOLIDSTATE PHYSICS), 2009, Vol. 44, pp. 621-633). But such consideration isnot appropriate. An alkali metal and an alkaline earth metal areunfavorable impurities for the oxide semiconductor layer 644 and shouldbe contained as little as possible. When an insulating film in contactwith the oxide semiconductor layer is an oxide, an alkali metal, inparticular, Na diffuses into the oxide and becomes Na⁺. In addition, Nacuts a bond between metal and oxygen or enters the bond in the oxidesemiconductor layer. As a result, transistor characteristics deteriorate(e.g., the transistor becomes normally-on (the shift of a thresholdvoltage to a negative side) or the mobility is decreased). Additionally,this also causes variation in characteristics of the transistor. Such aproblem is significant especially in the case where the hydrogenconcentration in the oxide semiconductor layer is extremely low.Therefore, the concentration of an alkali metal is strongly required toset in the above range in the case where the concentration of hydrogencontained in the oxide semiconductor is lower than or equal to 5×10¹⁹atoms/cm⁻³, particularly lower than or equal to 5×10¹⁸ atoms/cm⁻³.

Note that the hydrogen concentration and the alkali metal elementconcentration in the oxide semiconductor layer 644 are measured bysecondary ion mass spectroscopy (SIMS). Here, the oxide semiconductorlayer 644 is purified by sufficiently reducing the concentrations ofalkali metal element and hydrogen, and sufficiently supplied with oxygenso that defect states in an energy gap due to oxygen deficiency arereduced. The density of carriers generated due to a donor such ashydrogen and an alkali metal element in such an oxide semiconductorlayer 644 is lower than 1×10¹²/cm³, preferably lower than 1×10¹¹/cm³, orfurther preferably lower than 1.45×10¹⁰/cm³. In addition, for example,the off-state current (per unit channel width (1 μm), here) at roomtemperature (25° C.) is 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) or lessor 10 zA or less. With use of such an oxide semiconductor that is highlypurified to be intrinsic (i-type) or substantially intrinsic, thetransistor 662 can have excellent off-state current characteristics.

The capacitor 664 in FIG. 6 includes the electrode 642 b formed over theinsulating layer 628, the gate insulating layer 646, and an electrode649. That is, in the capacitor 664, the electrode 642 b functions as oneof electrodes, the electrode 649 functions as the other electrode, andthe gate insulating layer 646 functions as a dielectric.

An insulating layer 650 is formed over the transistor 662, and aninsulating layer 654 is formed over the insulating layer 650. Over theinsulating layer 654, a wiring 658 is formed. Here, the wiring 658 canbe a wiring to which the signal IN in the circuit illustrated in FIGS.1A and 1B is input.

The wiring 658 can be connected to an electrode 504 through an openingportion 501 formed in the insulating layer 654, the insulating layer650, and the gate insulating layer 646. Further, the electrode 504 isconnected to the metal compound region 624 a of the transistor 660 withan electrode 502 formed in an opening portion provided in the insulatinglayer 628. Thus, the wiring 658 is electrically connected to one of thesource and the drain of the transistor 660.

Note that the structure of the storage device according to an embodimentof the disclosed invention is not limited to that illustrated in FIG. 6.The details such as connection relations of the electrode and the likein the structure illustrated in FIG. 6 can be changed as appropriate.

For example, the structure illustrated in FIG. 6 is an example in whichthe oxide semiconductor layer 644 is provided below the electrode 642.However, the structure of the transistor is not limited thereto. Theoxide semiconductor layer 644 may be provided over the electrode 642.FIG. 7 illustrates an example in which the oxide semiconductor layer 644is provided over the electrode 642. Note that the same portions in FIG.7 as those in FIG. 6 are denoted by the same reference numerals.

In the structure illustrated in FIG. 7, end portions of the electrode642 a and the electrode 642 b preferably have tapered shapes. When theend portions of the electrode 642 a and the electrode 642 b have taperedportions, coverage with the oxide semiconductor layer 644 can beimproved and disconnection can be prevented, which is preferable. Here,a taper angle is, for example, greater than or equal to 30° and lessthan or equal to 60°. Note that the “taper angle” means an angle formedbetween the side surface and the bottom surface of a layer having atapered shape (for example, the electrode 642 a) when observed from adirection perpendicular to a cross section of the layer (a planeperpendicular to the substrate surface).

With a structure in which the whole of the oxide semiconductor layer 644overlaps with the gate electrode 648 or the wiring 658 (i.e., be coveredwith the gate electrode 648 or the wiring 658), entry of light from theabove into the oxide semiconductor layer 644 can be suppressed. Thus,light deterioration of the oxide semiconductor layer 644 can besuppressed.

Further, in the structures each illustrated in FIG. 6 and FIG. 7, thegate electrode 648 is provided over the oxide semiconductor layer 644.However, the structure is not limited thereto. The gate electrode 648may be provided below the oxide semiconductor layer 644. FIG. 8illustrates an example in which the gate electrode 648 is provided belowthe oxide semiconductor layer 644. Note that in FIG. 8, the sameportions as those in FIG. 6 or FIG. 7 are denoted by the same referencenumerals.

In FIG. 8, the electrode 642 a is connected to the electrode 503 in theopening portion provided in the gate insulating layer 646.

In the structure illustrated in FIG. 8, end portions of the gateelectrode 648 and the electrode 649 preferably have tapered shapes. Whenthe end portions the gate electrode 648 and the electrode 649 havetapered shapes, coverage with the gate insulating layer 646 is improved,which results in prevention of short circuit between the electrode 642 aand the gate electrode 648, short circuit between the electrode 642 band the gate electrode 648, short circuit between the electrode 642 band the electrode 649, and the like. Here, a taper angle is, forexample, greater than or equal to 30° and less than or equal to 60°.

Further, in the structure illustrated in FIG. 8, the oxide semiconductorlayer 644 may be provided over the electrode 642. FIG. 9 illustrates anexample of a structure different from that of FIG. 8, in that the oxidesemiconductor layer 644 is provided over the electrode 642. Note that inFIG. 9, the same portions as those in FIG. 6 to FIG. 8 are denoted bythe same reference numerals.

In the structures of each of FIG. 8 and FIG. 9, the gate electrode 648is provided below the oxide semiconductor layer 644. In such structures,the whole of the oxide semiconductor layer 644 overlaps with the gateelectrode 648, whereby entry of light from the lower portion into theoxide semiconductor layer 644 can be suppressed. Thus, lightdeterioration of the oxide semiconductor layer 644 can be suppressed.Furthermore, with a structure in which the whole of the oxidesemiconductor layer 644 overlaps with the wiring 658 (i.e., be coveredwith the wiring 658), entry of light from the above into the oxidesemiconductor layer 644 can be suppressed. Thus, light deterioration ofthe oxide semiconductor layer 644 can further be suppressed.

Further, in each of the structures illustrated in FIG. 6 and FIG. 8 (thestructure in which the electrode 642 a and the electrode 642 b areprovided over the oxide semiconductor layer 644), an oxide conductivelayer to be a source region and a drain region may be provided betweenthe oxide semiconductor layer 644 and the electrodes 642 a and 642 b.FIGS. 17A and 17B illustrate structures in which an oxide conductivelayer is further provided in the transistor 662 of FIG. 6. Note that inFIGS. 17A and 17B, components other than those included in thetransistor 662 are not illustrated.

In each of the transistors illustrated in FIGS. 17A and 17B, an oxideconductive layer 404 a and an oxide conductive layer 404 b whichfunctions as a source region and a drain region are formed between theoxide semiconductor layer 644 and the electrodes 642 a and 642 b. Shapesof the oxide semiconductor layer 404 a and the oxide semiconductor layer404 b in FIG. 17A are different from those of FIG. 17B in accordancewith a manufacturing process.

In the transistor of FIG. 17A, a stack of an oxide semiconductor filmand an oxide conductive film is formed, and then the stack of an oxidesemiconductor film and an oxide conductive film is processed through aphotography step, so that the island-shaped oxide semiconductor layer644 and an island-shaped oxide conductive film are concurrently formed.The electrode 642 a and the electrode 642 b are formed over the oxidesemiconductor layer and the oxide conductive film, and then, with use ofthe electrode 642 a and the electrode 642 b as a mask, the island-shapedoxide conductive layer is etched, so that the oxide conductive layer 404a and the oxide conductive layer 404 b which are to be a source regionand a drain region are formed.

In the transistor of FIG. 17B, the island-shaped oxide semiconductorlayer 644 is formed, an oxide conductive film is formed thereover, ametal conductive film is formed over the oxide conductive film, and theoxide conductive film and the metal conductive film are concurrentlyprocessed through one photolithography step, so that the oxideconductive layer 404 a, the oxide conductive layer 404 b, the electrode642 a, and the electrode 642 b are formed.

In order to prevent excessive etching of the oxide semiconductor layer644 in etching treatment for formation of the oxide conductive layer 404a and the oxide conductive layer 404 b, etching conditions (such as thekind of etchant, the concentration, and the etching time) are adjustedas appropriate.

A material of the oxide conductive layer preferably contains zinc oxideas a component and preferably does not contain indium oxide. For such anoxide conductive layer, zinc oxide, zinc aluminum oxide, zinc aluminumoxynitride, gallium zinc oxide, or the like can be used.

Contact resistance in the case where a metal electrode (such as amolybdenum electrode, a tungsten electrode, or the like) is in contactwith the oxide conductive layer can be lower than contact resistance inthe case where a metal electrode (such as a molybdenum electrode, atungsten electrode, or the like) is in contact with the oxidesemiconductor layer. Thus, provision of the above oxide conductive layerbetween the oxide semiconductor layer 644 and the electrodes 642 a and642 b enables reduction in contact resistance between the electrodes 642a and 642 b and the oxide conductive layer. Therefore, the resistance ofthe source and the drain can be reduced, so that high operation of thetransistor 662 can be achieved. In addition, the withstand voltage ofthe transistor 662 can be improved.

Further, in each of the structures illustrated in FIG. 6 to FIG. 9, thegate insulating layer 646 of the transistor 662 is used as a dielectriclayer of the capacitor 664; however, the structure is not limitedthereto. As the dielectric layer of the capacitor 664, an insulatinglayer different from the gate insulating layer 646 may be used. Inaddition, in each of the structures illustrated in FIG. 6 to FIG. 9, theelectrode 642 b functioning as the source electrode or the drainelectrode of the transistor 662 is used as one of a pair of electrodesof the capacitor 664; however, the structure is not limited thereto. Asone of a pair of electrodes of the capacitor 664, an electrode differentfrom the electrode 642 b, for example, an electrode formed in a layerdifferent from that of the electrode 642 b, may be used. Moreover, ineach of the structures illustrated in FIG. 6 to FIG. 9, the electrode649 formed in the same layer as the gate electrode 648 of the transistor662 is used as the other electrode of the capacitor 664; however, thestructure is not limited thereto. As the other electrode of thecapacitor 664, an electrode formed in a layer different from that of thegate electrode 648 may be used.

In each of the structures illustrated in FIG. 6 to FIG. 9, thetransistor 660 is formed in the semiconductor substrate. However, thestructure is not limited thereto. The transistor 660 may be formed usingan SOI substrate. Note that in general, the term “SOI substrate” means asubstrate where a silicon semiconductor layer is provided on aninsulating surface. In this specification and the like, the term “SOIsubstrate” also includes a substrate where a semiconductor layer formedusing a material other than silicon is provided over an insulatingsurface in its category. That is, a semiconductor layer included in the“SOI substrate” is not limited to a silicon semiconductor layer.Further, the transistor 660 may include a semiconductor layer formedusing silicon or the like formed over a substrate having an insulatingsurface. The semiconductor layer may be formed by crystallizing a thinlayer of an amorphous semiconductor formed over an insulating surface.

The methods, structures, and the like described in this embodiment canbe combined as appropriate with any of the methods and structuresdescribed in the other embodiments.

Embodiment 6

In this embodiment, a method for manufacturing a storage elementaccording to one embodiment of the disclosed invention will be describedwith reference to FIGS. 10A to 10E, FIGS. 11A to 11C, FIGS. 12A to 12D,FIGS. 13A to 13D, FIGS. 14A to 14D, and FIGS. 15A to 15D.

An example of a manufacturing method of a storage element illustrated inFIG. 6 is described. In the description below, first, a method formanufacturing the transistor 660 in the lower portion is described withreference to FIGS. 10A to 10E and FIGS. 11A to 11C, and then, a methodfor manufacturing the transistor 662 in the upper portion and thecapacitor 664 are described with reference to FIGS. 12A to 12D, FIGS.13A to 13D, FIGS. 14A to 14D, and FIGS. 15A to 15D.

<Manufacturing Method of Lower Transistor>

First, the substrate 600 including a semiconductor material is prepared(see FIG. 10A). As the substrate 600 including a semiconductor material,a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate made of silicon, silicon carbide, or the like; acompound semiconductor substrate made of silicon germanium or the like;an SOI substrate; or the like can be used. Here, an example of using asingle crystal silicon substrate as the substrate 600 including asemiconductor material is described. As the substrate 600 including asemiconductor material, in particular, a single crystal semiconductorsubstrate of silicon or the like is preferable because the speed of theread operation of the storage element can be increased.

Note that an impurity element imparting conductivity type may be addedto a region which later functions as the channel formation region 616 ofthe transistor 660, in order to control the threshold voltage of thetransistor. Here, an impurity element imparting conductivity is added sothat the threshold voltage of the transistor 660 becomes positive. Whenthe semiconductor material is formed using silicon, as the impurityimparting conductivity, for example, boron, aluminum, gallium, or thelike can be used. Note that it is preferable to perform heat treatmentafter adding an impurity element imparting conductivity, in order toactivate the impurity element or reduce defects generated in thesubstrate 600 during addition of the impurity element.

A protective layer 602 serving as a mask used for forming anelement-isolation insulating layer is formed over the substrate 600 (seeFIG. 10A). As the protective layer 602, an insulating layer formed usingsilicon oxide, silicon nitride, silicon oxynitride, or the like can beused, for example.

Next, etching of the substrate 600 is performed using the protectivelayer 602 as a mask, whereby part of the substrate 600 which is notcovered with the protective layer 602 (i.e., in an exposed region), isremoved (see FIG. 10B). As the etching, dry etching is preferablyperformed, but wet etching may be performed. An etching gas and anetchant can be selected as appropriate depending on a material of layersto be etched.

Then, an insulating layer is formed so as to cover the substrate 600,and the insulating layer is selectively removed, so that anelement-isolation insulating layer 606 is formed (see FIG. 10C). Theinsulating layer is formed using silicon oxide, silicon nitride, siliconoxynitride or the like. As a method for removing the insulating layer,any of etching treatment, polishing treatment such as chemicalmechanical polishing (CMP) treatment, and the like can be employed.Thus, a semiconductor region 604 which is isolated from othersemiconductor regions is formed. Note that after the etching of thesubstrate 600 using the protective layer 602 as a mask or after theformation of the element-isolation insulating layer 606, the protectivelayer 602 is removed.

Next, an insulating layer is formed over a surface of the semiconductorregion 604, and a layer including a conductive material is formed overthe insulating layer.

The insulating layer serves as a gate insulating layer later, and can beformed by heat treatment (thermal oxidation treatment, thermalnitridation treatment, or the like) of the surface of the semiconductorregion 604, for example. Instead of heat treatment, high-density plasmatreatment may be employed. The high-density plasma treatment can beperformed using, for example, a mixed gas of a rare gas such as He, Ar,Kr, or Xe and a gas such as oxygen, nitrogen oxide, ammonia, nitrogen,or hydrogen. Needless to say, the insulating layer may be formed using aCVD method, a sputtering method, or the like. The insulating layerpreferably has a single-layer structure or a stacked-layer structureincluding a film which contains any of silicon oxide, siliconoxynitride, silicon nitride, hafnium oxide, aluminum oxide, tantalumoxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)),hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)) to which nitrogen is added,hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)) to which nitrogen is added,and the like. The insulating layer can have a thickness, for example,greater than or equal to 1 nm and less than or equal to 100 nm,preferably greater than or equal to 10 nm and less than or equal to 50nm.

The layer including a conductive material can be formed using a metalmaterial such as aluminum, copper, titanium, tantalum, or tungsten. Thelayer including a conductive material may be formed using asemiconductor material such as polycrystalline silicon. There is noparticular limitation on a method for forming the layer including aconductive material, and any of a variety of film formation methods suchas an evaporation method, a CVD method, a sputtering method, or a spincoating method can be employed. Note that this embodiment shows anexample of the case where the layer including a conductive material isformed using a metal material.

After that, the insulating layer and the layer including a conductivematerial are selectively etched, so that the gate insulating layer 608and the gate electrode 610 are formed (see FIG. 10D).

Next, the channel formation region 616 and the impurity regions 620 aand 620 b are formed by adding phosphorus (P), arsenic (As), or the liketo the semiconductor region 604 (see FIG. 10E). Here, the transistor 660is an n-channel transistor. In order to form an re-channel transistor,an impurity element imparting conductivity type such as phosphorus orarsenic is added to the semiconductor region 604. If the transistor 660is a p-channel transistor, an impurity element imparting conductivitytype such as boron (B) or aluminum (Al) may be added to thesemiconductor region 604, so that the channel formation region 616 andthe impurity regions 620 a and 620 b are formed. The concentration ofthe impurity element imparting conductivity type to be added can be setas appropriate. In the case of the transistor 660 is highlyminiaturized, the concentration is preferably set to high.

Note that a sidewall insulating layer may be formed on the periphery ofthe gate electrode 610, so that a plurality of impurity regions whichhave different concentrations of an added impurity element impartingconductivity (e.g., a high-concentration impurity region which does notoverlap with the sidewall insulating layer and a low-concentrationimpurity region which overlaps with the sidewall insulating layer) areformed in the semiconductor region 604.

Then, a metal layer 622 is formed so as to cover the gate electrode 610,and the impurity regions 620 a and 620 b (see FIG. 11A). Any of avariety of film formation methods such as a vacuum evaporation method, asputtering method, or a spin coating method can be employed for formingthe metal layer 622. The metal layer 622 is preferably formed using ametal material that reacts with a semiconductor material included in thesemiconductor region 604 to form a low-resistance metal compound.Examples of such metal materials include titanium, tantalum, tungsten,nickel, cobalt, and platinum.

Next, heat treatment is performed so that the metal layer 622 reactswith the semiconductor material on a surface of the semiconductor region604. As a result, the metal compound region 624 a and the metal compoundregion 624 b which are in contact with the impurity region 620 a and theimpurity region 620 b are formed (see FIG. 11A). Note that when the gateelectrode 610 is formed using polycrystalline silicon or the like, ametal compound region is also formed in a region of the gate electrode610 in contact with the metal layer 622. The above metal compound regionhas sufficiently high conductivity. The formation of the metal compoundregions can properly reduce electric resistance of the source and drainand the like and improve element characteristics of the transistor 660.

As the heat treatment, irradiation with a flash lamp can be employed,for example. Although it is needless to say that another heat treatmentmethod may be used, a method by which heat treatment for an extremelyshort time can be achieved is preferably used in order to improve thecontrollability of chemical reaction in formation of the metal compound.Note that the metal layer 622 is removed after the metal compoundregions 624 a and 624 b are formed.

Through the above steps, the transistor 660 using the substrate 600including a semiconductor material is formed (see FIG. 11B). The thusformed transistor 660 can operate at high speed. Therefore, with use ofthe transistor 660, the storage element can read data at high speed.

Then, the insulating layer 628 is formed so as to cover the transistor660 formed in the above steps (see FIG. 11C). The insulating layer 628can be formed using a material including an inorganic insulatingmaterial such as silicon oxide, silicon oxynitride, silicon nitride, oraluminum oxide. It is particularly preferable to use a low dielectricconstant (low-k) material for the insulating layer 628 becausecapacitance due to overlap of electrodes or wirings can be sufficientlyreduced. Note that a porous insulating layer with such a material may beemployed as the insulating layer 628. A porous insulating layer has alower dielectric constant than an insulating layer with high density,and thus allows a further reduction in capacitance generated byelectrodes or wirings. Moreover, the insulating layer 628 can be formedusing an organic insulating material such as polyimide or acrylic. Notethat the insulating layer 628 has a single-layer structure in thisembodiment; however, an embodiment of the disclosed invention is notlimited to this. The insulating layer 628 may have a stacked structureof two or more layers. For example, a stacked structure including alayer formed using an organic insulating material and a layer formedusing an inorganic material may be used.

In the insulating layer 628, opening portions reaching the metalcompound region 624 a and the metal compound region 624 b are formed,and the electrode 502 and the electrode 503 are formed using aconductive layer. The conductive layer can be formed by a PVD methodsuch as a sputtering method, or a CVD method such as a plasma CVDmethod. As a material for the conductive layer, an element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, or tungsten;an alloy containing any of these elements as a component; or the likecan be used. Further, one or more materials selected from manganese,magnesium, zirconium, beryllium, neodymium, and scandium may be used.The conductive layer can have a single-layer structure or a stackedstructure including two or more layers. For example, the conductivelayer can have a single-layer structure of a titanium film or a titaniumnitride film, a single-layer structure of an aluminum film containingsilicon, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a titanium nitride film, or a three-layer structure inwhich a titanium film, an aluminum film, and a titanium film are stackedin this order.

Then, as treatment before formation of the transistor 662 and thecapacitor 664, CMP treatment is subjected to a surface of the insulatinglayer 628 (see FIG. 11C). Instead of CMP treatment, etching treatment orthe like can be employed. Note that in order to improve characteristicsof the transistor 662, the surfaces of the insulating layer 628, theelectrode 502, and the electrode 503 are preferably made as flat aspossible. For example, the surface of the insulating layer 628preferably has a root-mean-square (RMS) roughness of 1 nm or less.

Note that an electrode, a wiring, a semiconductor layer, an insulatinglayer may be further formed before and after the above steps describedwith FIGS. 10A to 10E and FIGS. 11A to 11C. In addition, a multilayerwiring structure in which an insulating layer and a conductive layer arestacked may be employed as a wiring structure, so that ahighly-integrated storage element can be realized.

<Manufacturing Method of Transistor in Upper Portion>

Next, manufacturing methods of the transistor 662 in an upper portionand the capacitor 664 are described. A manufacturing method of astructure corresponding to that illustrated in FIG. 6 is described withreference to FIGS. 12A to 12D. A manufacturing method of a structurecorresponding to that illustrated in FIG. 7 is described with referenceto FIGS. 13A to 13D. A manufacturing method of a structure correspondingto that illustrated in FIG. 8 is described with reference to FIGS. 14Ato 14D. A manufacturing method of a structure corresponding to thatillustrated in FIG. 9 is described with reference to FIGS. 15A to 15D.

First, the manufacturing method of a structure corresponding to thatillustrated in FIG. 6 is described with reference to FIGS. 12A to 12D.

An oxide semiconductor layer is formed over the insulating layer 628,the electrode 502, and the electrode 503, and the oxide semiconductorlayer is selectively etched to form the oxide semiconductor layer 644(see FIG. 12A).

An oxide semiconductor used in the oxide semiconductor layer 644preferably contains at least indium (In) or zinc (Zn). In particular, Inand Zn are preferably contained. As a stabilizer for reducing changes inelectrical characteristics of a transistor including the oxidesemiconductor, gallium (Ga) is preferably additionally contained. Tin(Sn) is preferably contained as a stabilizer. Hafnium (Hf) is preferablycontained as a stabilizer. Aluminum (Al) is preferably contained as astabilizer.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium(Lu) may be contained.

As the oxide semiconductor, for example, the following oxide can beused: an indium oxide, a tin oxide, a zinc oxide; a two-component metaloxide such as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-basedoxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide,or an In—Ga-based oxide; a three-component metal oxide such as anIn—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-basedoxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, anAl—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide,an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-basedoxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, anIn—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide,an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-basedoxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or anIn—Lu—Zn-based oxide; a four-component metal oxide such as anIn—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide.

Note that, for example, an In—Ga—Zn-based oxide means an oxidecontaining In, Ga, and Zn, and there is no limitation on the compositionratio of In, Ga, and Zn. The In—Ga—Zn-based oxide may contain a metalelement other than the In, Ga, and Zn.

For example, an In—Ga—Zn—O-based material with an atomic ratio whereIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or anyof oxides whose composition is in the neighborhood of the abovecompositions can be used. Alternatively, an In—Sn—Zn—O-based materialwith an atomic ratio where In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3(=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or any of oxides whosecomposition is in the neighborhood of the above compositions may beused.

However, the composition is not limited to those described above, and amaterial having an appropriate composition may be used in accordancewith necessary semiconductor characteristics (such as mobility,threshold voltage, and variation). In order to obtain necessarysemiconductor characteristics, it is preferable that the carrierconcentration, the impurity concentration, the defect density, theatomic ratio of a metal element to oxygen, the interatomic distance, thedensity, and the like be set to be appropriate.

For example, with the In—Sn—Zn—O-based material, a high mobility can berelatively easily obtained. Also in the case of using theIn—Ga—Zn—O-based material, the mobility can be increased by reducing thedefect density in the bulk.

Note that for example, the expression “the composition of an oxidecontaining In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),is in the neighborhood of the composition of an oxide containing In, Ga,and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b,and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r², and r maybe 0.05, for example. The same applies to other oxides.

The oxide semiconductor may be either single crystal ornon-single-crystal. In the latter case, the oxide semiconductor may beeither amorphous or polycrystal. Further, the oxide semiconductor mayhave either an amorphous structure including a portion havingcrystallinity or a non-amorphous structure.

In an oxide semiconductor in an amorphous state, a flat surface can beobtained with relative ease, so that when a transistor is manufacturedwith use of the oxide semiconductor with an amorphous structure,interface scattering can be reduced, and relatively high mobility can beobtained with relative ease.

In an oxide semiconductor having crystallinity, defects in a bulk can befurther reduced and when a surface flatness is improved, mobility higherthan that of an oxide semiconductor layer in an amorphous state can beobtained. In order to improve the surface flatness, the oxidesemiconductor is preferably formed over a flat surface. Specifically,the oxide semiconductor may be formed over a surface with the averagesurface roughness (Ra) of less than or equal to 1 nm, preferably lessthan or equal to 0.3 nm, further preferably less than or equal to 0.1nm.

Note that, Ra is obtained by expanding, into three dimensions, centerline average roughness that is defined by JIS B 0601 so as to be appliedto a surface. Ra can be expressed as an “average value of the absolutevalues of deviations from a reference surface to a designated surface”and is defined by the following formula.

$\begin{matrix}{{Ra} = {\frac{1}{S_{0}}{\int_{x_{2}}^{x_{1}}{\int_{y_{2}}^{y_{1}}{{{{f\left( {x,y} \right)} - Z_{0}}}\ {\mathbb{d}x}\ {\mathbb{d}y}}}}}} & \left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In the above formula, S₀ represents the area of a measurement surface (arectangular region which is defined by four points represented bycoordinates (x₁,y₁), (x₁,y₂), (x₂,y₁), and (x₂,y₂)), and Z₀ representsthe average height of the measurement surface. Ra can be measured usingan atomic force microscope (AFM).

As a typical example of the In—Ga—Zn—O-based oxide semiconductormaterial, one represented by InGaO₃ (ZnO)_(m) (m>0) is given. Using Minstead of Ga, there is an oxide semiconductor material represented byInMO₃(ZnO)_(m) (m>0). Here, M denotes one or more metal elementsselected from gallium (Ga), aluminum (Al), iron (Fe), nickel (Ni),manganese (Mn), cobalt (Co), or the like. For example, M may be Ga, Gaand Al, Ga and Fe, Ga and Ni, Ga and Mn, Ga and Co, or the like. Notethat the above-described compositions are derived from the crystalstructures that the oxide semiconductor material can have and are onlyexamples.

As a target used for forming the oxide semiconductor layer 644 by asputtering method, a target having a composition ratio expressed by theequation In:Ga:Zn=1:x:y (x is 0 or more, and y is 0.5 to 5 inclusive) ispreferable. For example, a target having a composition ratio,In:Ga:Zn=1:1:1 [atomic ratio] (x=1, y=1) (that is, In₂O₃:Ga₂O₃:ZnO=1:1:2[molar ratio]), can be used. Further, it is possible to use a targetwith a composition ratio, In:Ga:Zn=1:1:0.5 [atomic ratio], a target witha composition ratio, In:Ga:Zn=1:1:2 [atomic ratio], or a target with acomposition ratio, In:Ga:Zn=1:0:1 [atomic ratio] (x=0, y=1). Therelative density of the metal oxide in the metal oxide target is 80% orhigher, preferably 95% or higher, further preferably 99.9% or higher.The use of a metal oxide target having high relative density makes itpossible to form the oxide semiconductor layer 644 with a densestructure.

Alternatively, the oxide semiconductor layer 644 can be formed using anIn—Sn—Zn-based oxide. An In—Sn—Zn-based oxide can be referred to asITZO. For ITZO, an oxide target having a composition ratio,In:Sn:Zn=1:2:2, 2:1:3, 1:1:1, or 20:45:35 in an atomic ratio, is used,for example.

The atmosphere in which the oxide semiconductor layer 644 is formed ispreferably a rare gas (typically argon) atmosphere, an oxygenatmosphere, or a mixed atmosphere including a rare gas (typically argon)and oxygen. Specifically, it is preferable to use, for example, anatmosphere of a high-purity gas from which an impurity such as hydrogen,water, a hydroxyl group, or a hydride is removed so that the impurityconcentration is 1 ppm or lower (preferably the impurity concentrationis 10 ppb or lower).

In forming the oxide semiconductor layer 644, for example, an object tobe processed is held in a treatment chamber kept under reduced pressureand the object to be processed is heated to a temperature higher than orequal to 100° C. and lower than 550° C., preferably higher than or equalto 200° C. and lower than or equal to 400° C. Alternatively, thetemperature of the object in the formation of the oxide semiconductorlayer 644 may be room temperature (25° C.±10° C.). Then, moisture in thetreatment chamber is removed, a sputtering gas from which hydrogen,water, or the like have been removed is introduced, and the above targetis used, so that the oxide semiconductor layer 644 is formed. The oxidesemiconductor layer 644 is formed while the object is heated, wherebyimpurities in the oxide semiconductor layer 644 can be reduced. Inaddition, damage on the oxide semiconductor layer 644 due to sputteringcan be reduced. In order to remove moisture in the treatment chamber, anentrapment vacuum pump is preferably used. For example, a cryopump, anion pump, a titanium sublimation pump, or the like can be used. A turbopump provided with a cold trap may be used. By performing evacuationwith use of a cryopump or the like, hydrogen, water, or the like can beremoved from the treatment chamber; thus, the concentration of animpurity in the oxide semiconductor layer 644 can be reduced.

The oxide semiconductor layer 644 can be formed under the followingconditions, for example: the distance between the object to be processedand the target is 170 mm; the pressure is 0.4 Pa; the direct-current(DC) power is 0.5 kW; and the atmosphere is an oxygen (the proportion ofthe oxygen flow is 100%) atmosphere, an argon (the proportion of theargon flow is 100%) atmosphere, or a mixed atmosphere of oxygen andargon. Note that a pulse direct-current (DC) power supply is preferablyused in the formation of the oxide semiconductor layer because dust(e.g., powdery substances produced at the time of deposition) can bereduced and the film thickness of the oxide semiconductor layer 644 canbe uniform.

Note that before the oxide semiconductor layer 644 is formed by asputtering method, reverse sputtering in which plasma is generated withan argon gas introduced may be performed so that a material attached toa surface where the oxide semiconductor layer is to be formed (e.g., asurface of the insulating layer 628) is removed. Here, the reversesputtering is a method by which ions collide with a surface to beprocessed so that the surface is modified, in contrast to normalsputtering by which ions collide with a sputtering target. An example ofa method for making ions collide with a surface to be processed is amethod in which high-frequency voltage is applied to the surface to beprocessed in an argon atmosphere so that plasma is generated in thevicinity of the object to be processed. Note that an atmosphere ofnitrogen, helium, oxygen, or the like may be used instead of an argonatmosphere.

In this embodiment, the oxide semiconductor layer 644 is formed by asputtering method using an In—Ga—Zn—O-based metal oxide target. Thethickness of the oxide semiconductor layer 644 is greater than or equalto 1 nm and less than or equal to 50 nm, preferably, greater than orequal to 2 nm and less than or equal to 20 nm, further preferably,greater than or equal to 3 nm and less than or equal to 15 nm. However,the appropriate thickness differs depending on an oxide semiconductormaterial or the like, and thus, the thickness of the oxide semiconductorlayer 644 can be selected depending on a material to be used or thelike. Note that the surface of the insulating layer 628 is made as flatas possible as described above, whereby even if the oxide semiconductorlayer 644 has a small thickness, a cross-sectional of the portioncorresponding to the channel formation region of the oxide semiconductorlayer 644 can be made flat. By making the cross-sectional shape of theportion corresponding to the channel formation region in the oxidesemiconductor layer 644 flat, leakage current of the transistor 662 canbe reduced as compared to the case where the cross-sectional shape ofthe oxide semiconductor layer 644 is not flat.

After formation of the oxide semiconductor layer 644, a heat treatment(first heat treatment) is preferably performed on the oxidesemiconductor layer 644. Excessive hydrogen (including water and ahydroxyl group) in the oxide semiconductor layer 644 can be removed bythe first heat treatment. The temperature of the first heat treatmentis, for example, higher than or equal to 300° C. and lower than 550° C.,preferably higher than or equal to 400° C. and lower than or equal to500° C.

The first heat treatment can be performed in such a manner that, forexample, a process object is introduced into an electric furnace inwhich a resistance heating element or the like is used and heated at450° C. in a nitrogen atmosphere for an hour. During the heat treatment,the oxide semiconductor layer is not exposed to the atmosphere toprevent the entry of water and hydrogen.

The heat treatment apparatus is not limited to the electric furnace andmay be an apparatus for heating an object by thermal radiation orthermal conduction from a medium such as a heated gas. For example, anRTA (rapid thermal anneal) apparatus such as a GRTA (gas rapid thermalanneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus canbe used. An LRTA apparatus is an apparatus for heating an object to beprocessed by radiation of light (an electromagnetic wave) emitted from alamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, acarbon arc lamp, a high pressure sodium lamp, or a high pressure mercurylamp. A GRTA apparatus is an apparatus for performing heat treatmentusing a high-temperature gas. As the gas, an inert gas that does notreact with an object to be processed by heat treatment, for example,nitrogen or a rare gas such as argon is used.

For example, as the first heat treatment, a GRTA process may beperformed as follows. The object is put in an inert gas atmosphere thathas been heated, heated for several minutes, and taken out from theinert gas atmosphere. The GRTA process enables high-temperature heattreatment for a short time. Moreover, the GRTA process can be employedeven when the temperature exceeds the upper temperature limit of theobject.

Note that the inert gas may be switched to a gas including oxygen duringthe process. This is because defect level in energy gap due to oxygendeficiency can be reduced by performing the first heat treatment in anatmosphere including oxygen. In addition, it is preferable that theoxide semiconductor layer 644 become to contain excessive oxygen. Theoxygen excessively contained exists between lattices in the oxidesemiconductor layer 644. Note that as the inert gas atmosphere, anatmosphere that contains nitrogen or a rare gas (e.g., helium, neon, orargon) as its main component and does not contain water, hydrogen, orthe like is preferably used. For example, the purity of nitrogen or arare gas such as helium, neon, or argon introduced into a heat treatmentapparatus is greater than or equal to 6 N (99.9999%), preferably greaterthan or equal to 7 N (99.99999%) (that is, the concentration of theimpurities is less than or equal to 1 ppm, preferably less than or equalto 0.1 ppm).

The above heat treatment (first heat treatment) can be referred to asdehydration treatment, dehydrogenation treatment, or the like because ofits effect of removing hydrogen, water, and the like. Such heattreatment can also be performed at the following timing: after theformation of the oxide semiconductor layer, after the formation of thegate insulating layer 646 formed later, after the formation of the gateelectrode 648, or the like. Such heat treatment may be conducted once orplural times.

The oxide semiconductor layer may be etched either before or after theheat treatment. In view of miniaturization of elements, dry etching ispreferably used; however, wet etching may be used. An etching gas and anetchant can be selected as appropriate depending on a material of layersto be etched.

Next, a conductive layer is formed over the oxide semiconductor layer644 and is selectively etched to form the electrode 642 a, the electrode642 b, and the electrode 504 (see FIG. 12B). Note that the electrode 642a is provided to be connected to the electrode 503. The electrode 504 isprovided to be connected to the electrode 502.

The conductive layer can be formed by a PVD method such as a sputteringmethod, or a CVD method such as a plasma CVD method. As a material forthe conductive layer, an element selected from aluminum, chromium,copper, tantalum, titanium, molybdenum, or tungsten; an alloy containingany of these elements as a component; or the like can be used. Further,one or more materials selected from manganese, magnesium, zirconium,beryllium, neodymium, and scandium may be used.

The conductive layer can have a single-layer structure or a stackedstructure including two or more layers. For example, the conductivelayer can have a single-layer structure of a titanium film or a titaniumnitride film, a single-layer structure of an aluminum film containingsilicon, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a titanium nitride film, or a three-layer structure inwhich a titanium film, an aluminum film, and a titanium film are stackedin this order. Note that the conductive layer having a single-layerstructure of a titanium film or a titanium nitride film has an advantagein that it can be easily processed into the electrodes 642 a and 642 bhaving a tapered shape.

Alternatively, the conductive layer may be formed using conductive metaloxide. As the conductive metal oxide, indium oxide (In₂O₃), tin oxide(SnO₂), zinc oxide (ZnO), an indium oxide-tin oxide alloy (In₂O₃—SaO₂,which is abbreviated to ITO in some cases), an indium oxide-zinc oxidealloy (In₂O₃—ZnO), or any of these metal oxide materials in whichsilicon or silicon oxide is included can be used.

Although either dry etching or wet etching may be performed as theetching of the conductive layer, dry etching with high controllabilityis preferably used for miniaturization. Further, the etching may beperformed so that end portions of the electrodes 642 a and 642 b are tobe formed have a tapered shape. The taper angle can be, for example,greater than or equal to 30° and less than or equal to 60°.

The channel length (L) of the transistor 662 in the upper portion isdetermined by a distance between a lower end portion of the electrode642 a and a lower end portion of the electrode 642 b. When lightexposure is performed to form a mask used for forming a transistor witha channel length (L) of less than 25 nm, it is preferable to use extremeultraviolet light with a short wavelength of several nanometers toseveral tens of nanometers. In the light exposure with extremeultraviolet light, the resolution is high and the focal depth is large.Accordingly, the channel length (L) of the transistor 662 formed latercan be less than 2 μm, preferably greater than or equal to 10 nm andless than or equal to 350 nm (0.35 μm), whereby the operation speed ofthe circuit can be increased.

The electrode 642 b is to be one of a pair of electrodes of thecapacitor 664.

Note that an insulating layer functioning as a base of the transistor662 may be provided over the insulating layer 628. The insulating layercan be formed by a PVD method, a CVD method, or the like.

Next, the gate insulating layer 646 is formed to cover the electrode 642a, the electrode 642 b, the electrode 504, and the oxide semiconductorlayer 644 (see FIG. 12C).

The gate insulating layer 646 can be formed by a CVD method, asputtering method, or the like. The gate insulating layer 646 ispreferably formed by a method in which hydrogen is sufficiently reducedbecause the gate insulating layer 646 is to be in contact with the oxidesemiconductor layer 644. The gate insulating layer 646 preferablyincludes silicon oxide, silicon nitride, silicon oxynitride, aluminumoxide, tantalum oxide, hafnium oxide, yttrium oxide, hafnium silicate(HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate (HfSi_(x)O_(y) (x>0, y>0))to which nitrogen is added, hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0))to which nitrogen is added, or the like. The gate insulating layer 646may have a single-layer structure or a stacked structure. There is noparticular limitation on the thickness of the gate insulating layer 646,but in the case where the storage element is miniaturized, the gateinsulating layer 646 is formed thin. For example, in the case of usingsilicon oxide as the gate insulating layer 646, the thickness of thegate insulating layer 646 can be greater than or equal to 1 nm and lessthan or equal to 100 nm, preferably greater than or equal to 10 nm andless than or equal to 50 nm.

When the gate insulating layer 646 is thin as in the above description,a problem of gate leakage of the transistor 662 due to a tunnelingeffect or the like is caused. In order to solve the problem of gateleakage, it is preferable that the gate insulating layer 646 be formedusing a high dielectric constant (high-k) material such as hafniumoxide, tantalum oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y)(x>0, y>0)), hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)) to whichnitrogen is added, or hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)) towhich nitrogen is added. By using a high-k material for the gateinsulating layer 646, the thickness thereof can be increased forsuppression of gate leakage where favorable electric characteristics aremaintained. For example, the relative permittivity of hafnium oxide isapproximately 15, which is much higher than that of silicon oxide whichis 3 to 4. With such a material, the gate insulating layer 646 where theequivalent oxide thickness is less than 15 nm, preferably 2 nm to 10 nm,can be easily formed. Note that a stacked structure of a film containinga high-k material and a film containing any one of silicon oxide,silicon nitride, silicon oxynitride, silicon nitride oxide, aluminumoxide, and the like may be employed.

After formation of the gate insulating layer 646, a second heattreatment is preferably performed in an oxygen atmosphere. Thetemperature of the heat treatment is set in the range of 200° C. to 450°C., preferably 250° C. to 350° C. By the second heat treatment, oxygenis supplied to the oxide semiconductor layer 644.

Note that in this embodiment, the second heat treatment is performedafter the gate insulating layer 646 is formed; the timing of the secondheat treatment is not limited thereto. For example, the second heattreatment may be performed after the gate electrode 648 is formed.Alternatively, the second heat treatment may be performed following thefirst heat treatment, the first heat treatment may double as the secondheat treatment, or the second heat treatment may double as the firstheat treatment.

Instead of performing the second heat treatment in an oxygen atmosphere,a layer containing oxygen is formed as an insulating layer (e.g., thegate insulating layer 646) adjacent to the oxide semiconductor layer644, and then a heat treatment is performed in a nitrogen atmosphere orthe like, whereby oxygen is supplied from the insulating layer to theoxide semiconductor layer 644.

Further, instead of performing the second heat treatment in an oxygenatmosphere, oxygen may be added to the oxide semiconductor layer 644 bydoping.

As described above, after the dehydration treatment or dehydrogenationtreatment, oxygen is supplied to the oxide semiconductor layer 644,whereby defect level in the energy gap due to oxygen deficiency in theoxide semiconductor layer 644 can be reduced. Note that it is preferablethat the oxide semiconductor layer 644 be made to contain oxygenexcessively. Oxygen contained excessively exists between lattices in theoxide semiconductor layer 644.

The gate insulating layer 646 functions as a dielectric layer of thecapacitor 664.

Note that an insulating layer (corresponding to, for example, the gateinsulating layer 646 and the insulating layer 628 in the structureillustrated in FIG. 6 and FIG. 7, and the gate insulating layer 646 andthe insulating layer 650 in the structures illustrated in FIG. 8 andFIG. 9) in contact with the oxide semiconductor layer 644 is preferablyformed using an insulating material including a Group 13 element andoxygen. Many of oxide semiconductor materials include a Group 13element, and an insulating material including a Group 13 element workswell with an oxide semiconductor. By using such an insulating materialfor an insulating layer in contact with the oxide semiconductor, thecondition of an interface between the oxide semiconductor and theinsulating layer can keep favorable.

An insulating material including a Group 13 element refers to aninsulating material including one or more Group 13 elements. As theinsulating material containing a Group 13 element, gallium oxide,aluminum oxide, aluminum gallium oxide, and gallium aluminum oxide canbe given, for example. Here, aluminum gallium oxide refers to a materialin which the amount of aluminum (at. %) is larger than that of gallium(at. %), and gallium aluminum oxide refers to a material in which theamount of gallium (at. %) is larger than or equal to that of aluminum(at. %).

For example, in the case where an insulating layer is formed to be incontact with the oxide semiconductor layer 644 containing gallium, amaterial containing gallium oxide may be used for the insulating layer,so that favorable characteristics can be kept at the interface betweenthe oxide semiconductor layer 644 and the insulating layer. For example,when the oxide semiconductor layer 644 and the insulating layercontaining gallium oxide are provided in contact with each other, pileupof hydrogen at the interface between the oxide semiconductor layer 644and the insulating layer can be reduced. Note that a similar effect canbe obtained in the case where an element in the same group as aconstituent element of the oxide semiconductor is used in the insulatinglayer. For example, it is effective to form the insulating layer withuse of a material including aluminum oxide. Note that aluminum oxide isimpermeable property to water. Thus, it is preferable to use a materialincluding aluminum oxide in terms of preventing entry of water to theoxide semiconductor layer 644.

Further, it is preferable to perform a heat treatment in an oxygenatmosphere or oxygen doping so that part or the whole of the insulatinglayer which is in contact with the oxide semiconductor layer 644 is madeto contain oxygen whose proportion is higher than the stoichiometry ofthe insulating material of the insulating layer. “Oxygen doping” refersto addition of oxygen into a bulk. Note that the term “bulk” is used inorder to clarify that oxygen is added not only to a surface of a thinfilm but also to the inside of the thin film. In addition, “oxygendoping” includes “oxygen plasma doping” in which oxygen which is made tobe plasma is added to a bulk. The oxygen doping may be performed usingan ion implantation method or an ion doping method.

For example, in the case where the insulating layer in contact with theoxide semiconductor layer 644 is formed using gallium oxide, thecomposition of gallium oxide can be set to be Ga₂O_(x) (X=3+α, 0<α<1) byheat treatment in an oxygen atmosphere or oxygen doping.

In the case where the insulating layer in contact with the oxidesemiconductor layer 644 is formed using aluminum oxide, the compositionof aluminum oxide can be set to be Al₂O_(x) (X=3+α, 0<α<1) by heattreatment in an oxygen atmosphere or oxygen doping.

In the case where the insulating layer in contact with the oxidesemiconductor layer 644 is formed using gallium aluminum oxide (oraluminum gallium oxide), the composition of gallium aluminum oxide (oraluminum gallium oxide) can be set to be Ga_(X)Al_(2−X)O_(3+α), (0<X<20<α<1) by heat treatment in an oxygen atmosphere or oxygen doping.

By oxygen doping, an insulating layer which includes a region where theproportion of oxygen is higher than the stoichiometry of the insulatingmaterial of the insulating layer can be formed. When such an insulatinglayer is in contact with the oxide semiconductor layer, oxygen thatexists excessively in the insulating layer is supplied to the oxidesemiconductor layer, and oxygen deficiency in the oxide semiconductorlayer or at the interface between the oxide semiconductor layer and theinsulating layer is reduced. Thus, the oxide semiconductor layer can bean i-type or substantially i-type oxide semiconductor.

The insulating layer including a region where the proportion of oxygenis higher than the stoichiometry of the insulating material may beapplied to either of the insulating layers (the insulating layerpositioned over the oxide semiconductor layer and the insulating layerpositioned below the oxide semiconductor layer) in contact with theoxide semiconductor layer 644; however, it is preferable to apply suchan insulating layer to both of the insulating layers. Theabove-described effect can be enhanced with a structure where thesemiconductor layer 644 is sandwiched between the insulating layerswhich each include a region where the proportion of oxygen is higherthan the stoichiometry by providing the insulating layers to be incontact with the semiconductor layer 644 and to be located on the upperside and the lower side of the oxide semiconductor layer.

The insulating layers provided on the upper side and the lower side ofthe oxide semiconductor layer may include the same constituent elementsor different constituent elements. For example, the insulating layers onthe upper side and the lower side may be both formed of gallium oxidewhose composition is Ga₂O_(x) (X=3+α, 0<α<1). Alternatively, one of theinsulating layers on the upper side and the lower side may be formed ofGa₂O_(x) (X=3+α, 0<α<1) and the other may be formed of aluminum oxidewhose composition is Al₂O_(x) (x=3+α, 0<α<1).

The insulating layer in contact with the oxide semiconductor layer 644may be formed by stacking insulating layers each of which includes aregion where the proportion of oxygen is higher than the stoichiometry.For example, the insulating layer on the upper side of the oxidesemiconductor layer 644 may be formed as follows: gallium oxide whosecomposition is Ga₂O_(X) (X=3+α, 0<α<1) is formed and gallium aluminumoxide (aluminum gallium oxide) whose composition isGa_(X)Al_(2−X)O_(3+α) (0<X<2, 0<α<1) may be formed thereover. Note thatthe insulating layer on the lower side of the oxide semiconductor layer644 may be formed by stacking insulating layers each of which includes aregion where the proportion of oxygen is higher than the stoichiometry.Further, both of the insulating layers on the upper side and the lowerside of the oxide semiconductor layer 644 may be formed by stackinginsulating layers each of which includes a region where the proportionof oxygen is higher than the stoichiometry.

Next, the gate electrode 648 and the electrode 649 are formed over thegate insulating layer 646 (see FIG. 12C).

The gate electrode 648 and the electrode 649 can be formed in such amanner that a conductive layer is formed over the gate insulating layer646 and then etched selectively. The conductive layer to be the gateelectrode 648 and the electrode 649 can be formed by a PVD method suchas a sputtering method, or a CVD method such as a plasma CVD method. Thedetails of materials and the like are similar to those of the electrode642 a, the electrode 642 b, and the like; thus, the description thereofcan be referred to.

Further, the electrode 649 functions as the other electrode of a pair ofelectrodes of the capacitor 664.

Through the above steps, the transistor 662 including thehighly-purified oxide semiconductor layer 644 and the capacitor 664 arecompleted (see FIG. 12C). By the above manufacturing method, the oxidesemiconductor layer 644 from which the hydrogen concentration issufficiently reduced is highly purified, and oxygen is sufficientlysupplied thereto, so that defect level in the energy gap due to oxygendeficiency can be reduced. The thus formed oxide semiconductor layer 644is an intrinsic (i-type) or substantially intrinsic oxide semiconductor,and such an oxide semiconductor layer 644 is used for the channelformation region, whereby the transistor 662 can have excellentoff-state current characteristics.

Next, the insulating layer 650 and the insulating layer 654 are formedover the gate insulating layer 646, the gate electrode 648, and theelectrode 649 (see FIG. 12D). The insulating layer 650 and theinsulating layer 654 can be formed by a PVD method, a CVD method, or thelike. The insulating layer 650 and the insulating layer 654 can beformed so as to have a single-layer structure or a stacked structureusing a material including an inorganic insulating material such assilicon oxide, silicon oxynitride, silicon nitride, hafnium oxide, oraluminum oxide.

Note that the insulating layer 654 is preferably formed using a lowdielectric constant material or a low dielectric constant structure(such as a porous structure). The dielectric constant of the insulatinglayer 654 is reduced, whereby the capacitance generated between wiringsor electrodes can be reduced, which results in higher speed operation.

Note that the insulating layer 654 is preferably formed so as to have aflat surface. This is because a flat surface of the insulating layer 654makes it possible to form an electrode, a wiring, or the like preferablyover the insulating layer 654 even in the case where, for example, thestorage element is miniaturized. The insulating layer 654 can beplanarized using a method such as CMP (chemical mechanical polishing).

Next, the opening portion 501 reaching the electrode 504 is formed inthe insulating layer 650 and the insulating layer 654. Then, the wiring658 is formed (see FIG. 12D). The wiring 658 is formed in such a mannerthat a conductive layer is formed by a PVD method including a sputteringmethod or a CVD method such as a plasma CVD method and then theconductive layer is patterned. As a material for the conductive layer,an element selected from aluminum, chromium, copper, tantalum, titanium,molybdenum, and tungsten; an alloy containing any of these elements as acomponent; or the like can be used. Further, one or more materialsselected from manganese, magnesium, zirconium, beryllium, neodymium, andscandium may be used. The details are similar to those of the electrodes642 a and 642 b. Note that an electrode may be formed in the openingportion 501 and the wiring 658 is formed so as to be connected to theelectrode.

Through the above steps, the storage element having the structureillustrated in FIG. 6 can be manufactured.

Next, a manufacturing method of a structure corresponding to thatillustrated in FIG. 7 is described with reference to FIGS. 13A to 13D.

A difference between the manufacturing method illustrated in FIGS. 12Ato 12D and the manufacturing method illustrated in FIGS. 13A to 13D isformation methods of the oxide semiconductor layer 644 and the electrode642. In the manufacturing method illustrated in FIGS. 13A to 13D, themanufacturing method except for the formation methods of the oxidesemiconductor layer 644 and the electrode 642 is similar to themanufacturing method illustrated in FIGS. 12A to 12D; thus, thedescription thereof is omitted.

A conductive layer is formed over the electrode 502, the electrode 503,and the insulating layer 628 and selectively etched, so that theelectrode 642 a, the electrode 642 b, and the electrode 504 are formed(see FIG. 13A). The conductive layer can be formed using the materialand method which are similar to those of the conductive layer used forforming the electrode 642 a, the electrode 642 b, and the electrode 504;thus, the description of the conductive layer is omitted.

Next, the oxide semiconductor layer 644 is formed over the electrode 642a, the electrode 642 b, and the electrode 504 (see FIG. 13B). The oxidesemiconductor layer 644 can be formed using the material and methodwhich are similar to those used for forming the oxide semiconductorlayer 644 in the manufacturing method illustrated in FIGS. 12A to 12D;thus, the description thereof is omitted.

Next, the gate insulating layer 646 is formed so as to cover theelectrode 642 a, the electrode 642 b, the electrode 504, and the oxidesemiconductor layer 644 (see FIG. 13C). The following manufacturingsteps are similar to those illustrated in FIGS. 12A to 12D; thus,description thereof is omitted.

Through the above steps, the storage element having the structureillustrated in FIG. 7 can be manufactured.

Next, a manufacturing method of a structure corresponding to thatillustrated in FIG. 8 is described with reference to FIGS. 14A to 14D.

A difference between the manufacturing method illustrated in FIGS. 12Ato 12D and the manufacturing method illustrated in FIGS. 14A to 14D isformation methods of the gate electrode 648, the electrode 649, theelectrode 504, and the gate insulating layer 646. The other part of themethod in the manufacturing method illustrated in FIGS. 14A to 14D issimilar to the manufacturing method illustrated in FIGS. 12A to 12D;thus, the description thereof is omitted.

A conductive layer is formed over the electrode 502, the electrode 503,and the insulating layer 628, and selectively etched, so that the gateelectrode 648, the electrode 649, and the electrode 504 are formed (seeFIG. 14A). The conductive layer can be formed using the material andmethod which are similar to those of the conductive layer used forforming the gate electrode 648 and the electrode 649; thus, thedescription of the conductive layer is omitted.

Then, the gate insulating layer 646 is formed so as to cover the gateelectrode 648, the electrode 649, and the electrode 504 (see FIG. 14B).The gate insulating layer 646 can be formed using the material andmethod which are similar to those used for forming the gate insulatinglayer 646 in the manufacturing method illustrated in FIGS. 12A to 12D;thus, description thereof is omitted.

Then, the oxide semiconductor layer 644 is formed over the gateinsulating layer 646 (see FIG. 14B). The oxide semiconductor layer 644can be formed using the material and method which are similar to thoseused for forming the oxide semiconductor layer 644 in the manufacturingmethod illustrated in FIGS. 12A to 12D; thus, description thereof isomitted.

Then, a conductive layer is formed over the oxide semiconductor layer644, and selectively etched, so that the electrodes 642 a and 642 b areformed (see FIG. 14C). The conductive layer can be formed using thematerial and method which are similar to those of the conductive layerused for forming the electrodes 642 a and 642 b in the manufacturingmethod illustrated in FIGS. 12A to 12D; thus, description thereof isomitted.

The following manufacturing steps are similar to those illustrated inFIGS. 12A to 12D; thus, description thereof is omitted.

Through the above steps, the storage element illustrated in FIG. 8 canbe manufactured.

Next, a manufacturing method of a structure corresponding to thatillustrated in FIG. 9 is described with reference to FIGS. 15A to 15D.

A difference between the manufacturing method illustrated in FIGS. 14Ato 14D and the manufacturing method illustrated in FIGS. 15A to 15D isformation methods of the oxide semiconductor layer 644 and the electrode642. In the manufacturing method illustrated in FIGS. 15A to 15D, themanufacturing method except for formation of the oxide semiconductorlayer 644 and the electrode 642 is similar to the manufacturing methodillustrated in FIGS. 14A to 14D; thus, the description thereof isomitted.

A conductive layer is formed over the gate insulating layer 646 andselectively etched, so that the electrode 642 a and the electrode 642 bare formed (see FIG. 15B). The conductive layer can be formed using thematerial and method which are similar to those of the conductive layerused for forming the electrodes 642 a and 642 b in the manufacturingmethod illustrated in FIGS. 14A to 14D; description thereof is omitted.

Then, the oxide semiconductor layer 644 is formed over the electrodes642 a and 642 b (see FIG. 15C). The oxide semiconductor layer 644 can beformed using the material and method which are similar to those used forforming the oxide semiconductor layer 644 in the manufacturing methodillustrated in FIGS. 14A to 14D; description thereof is omitted.

The following manufacturing steps are similar to those illustrated inFIGS. 14A to 14D; thus, description thereof is omitted.

Through the above steps, the storage element having the structureillustrated in FIG. 9 can be manufactured.

Note that before or after the above steps described in FIGS. 12A to 12D,FIGS. 13A to 13D, FIGS. 14A to 14D, and FIGS. 15A to 15D, a step forforming an additional electrode, wiring, semiconductor layer, orinsulating layer may be performed. For example, a multilayer wiringstructure in which an insulating layer and a conductive layer arestacked may be employed as a wiring structure, so that ahighly-integrated storage element can be realized.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 7

An embodiment of the oxide semiconductor layer 644 of the transistor 662will be described with reference to FIGS. 16A to 16C.

The oxide semiconductor layer of this embodiment has a structureincluding a first crystalline oxide semiconductor layer and a secondcrystalline oxide semiconductor layer which is stacked over the firstcrystalline oxide semiconductor layer and is thicker than the firstcrystalline oxide semiconductor layer.

An insulating layer 437 is formed over the insulating layer 628. In thisembodiment, an oxide insulating layer with a thickness greater than orequal to 50 nm and less than or equal to 600 nm is formed as theinsulating layer 437 by a PCVD method or a sputtering method. As theoxide insulating layer, a single layer selected from a silicon oxidefilm, a gallium oxide film, an aluminum oxide film, a silicon nitridefilm, a silicon oxynitride film, an aluminum oxynitride film, and asilicon nitride oxide film or a stack of any of these films can be used.

Next, a first oxide semiconductor film with a thickness greater than orequal to 1 nm and less than or equal to 10 nm is formed over theinsulating layer 437. The first oxide semiconductor layer is formed by asputtering method, and the substrate temperature in the film formationby a sputtering method is set to be higher than or equal to 200° C. andlower than or equal to 400° C.

In this embodiment, the first oxide semiconductor film is formed to athickness of 5 nm in an oxygen atmosphere, an argon atmosphere, or anatmosphere including argon and oxygen under conditions where a targetfor an oxide semiconductor (a target for an In—Ga—Zn—O-based oxidesemiconductor including In₂O₃, Ga₂O₃, and ZnO at 1:1:2 [molar ratio]) isused, the distance between the substrate and the target is 170 mm, thesubstrate temperature is 250° C., the pressure is 0.4 Pa, and the directcurrent (DC) power is 0.5 kW.

Next, a first heat treatment is performed under a condition where theatmosphere of a chamber in which the substrate is set is an atmosphereof nitrogen or dry air. The temperature of the first heat treatment ishigher than or equal to 400° C. and lower than or equal to 750° C.Through the first heat treatment, a first crystalline oxidesemiconductor layer 450 a is formed (see FIG. 16A).

Depending on the substrate temperature at the time of deposition or thetemperature of the first heat treatment, the deposition and the firstheat treatment causes crystallization from a film surface and crystalgrows from the film surface toward the inside of the film; thus, c-axisaligned crystal is obtained. By the first heat treatment, large amountsof zinc and oxygen gather to the film surface, and one or more layers ofgraphene-type two-dimensional crystal including zinc and oxygen andhaving a hexagonal upper plane are formed at the outermost surface; thelayer or the layers at the outermost surface grow in the thicknessdirection to form a stack of layers. By increasing the temperature ofthe heat treatment, crystal growth proceeds from the surface to theinside and further from the inside to the bottom.

By the first heat treatment, oxygen in the insulating layer 437 that isan oxide insulating layer is diffused to an interface between theinsulating layer 437 and the first crystalline oxide semiconductor layer450 a or the vicinity of the interface (within ±5 nm from theinterface), whereby oxygen deficiency in the first crystalline oxidesemiconductor layer is reduced.

Next, a second oxide semiconductor film with a thickness more than 10 nmis formed over the first crystalline oxide semiconductor layer 450 a.The second oxide semiconductor film is formed by a sputtering method,and the substrate temperature in the film formation is set to be higherthan or equal to 200° C. and lower than or equal to 400° C. By settingthe substrate temperature in the film formation to be higher than orequal to 200° C. and lower than or equal to 400° C., precursors can bearranged in the second oxide semiconductor film formed over and incontact with the surface of the first crystalline oxide semiconductorlayer and so-called orderliness can be obtained.

In this embodiment, the second oxide semiconductor film is formed to athickness of 25 nm in an oxygen atmosphere, an argon atmosphere, or anatmosphere including argon and oxygen under conditions where a targetfor an oxide semiconductor (a target for an In—Ga—Zn-based oxidesemiconductor including In₂O₃, Ga₂O₃, and ZnO at 1:1:2 [molar ratio]) isused, the distance between the substrate and the target is 170 mm, thesubstrate temperature is 400° C., the pressure is 0.4 Pa, and the directcurrent (DC) power is 0.5 kW.

Then, a second heat treatment is performed under a condition where theatmosphere of a chamber in which the substrate is set is a nitrogenatmosphere or a dry air. The temperature of the second heat treatment ishigher than or equal to 400° C. and lower than or equal to 750° C. Bythe second heat treatment, a second crystalline oxide semiconductorlayer 450 b is formed (see FIG. 16B). The second heat treatment can beperformed in a nitrogen atmosphere, an oxygen atmosphere, or a mixedatmosphere of nitrogen and oxygen. By the second heat treatment, crystalgrowth proceeds in the thickness direction with use of the firstcrystalline oxide semiconductor layer 450 a as a nucleus, that is,crystal growth proceeds from the bottom to the inside; thus, the secondcrystalline oxide semiconductor layer 450 b is formed.

It is preferable that steps from the formation of the insulating layer437 to the second heat treatment be successively performed withoutexposure to the air. The steps from the formation of the insulatinglayer 437 to the second heat treatment are preferably performed in anatmosphere which is controlled to include little hydrogen and moisture(such as an inert atmosphere, a reduced-pressure atmosphere, or adry-air atmosphere); in terms of moisture, for example, a dry nitrogenatmosphere with a dew point of −40° C. or lower, preferably a dew pointof −50° C. or lower may be employed.

Next, the stack of the oxide semiconductor layers, the first crystallineoxide semiconductor layer 450 a and the second crystalline oxidesemiconductor layer 450 b, is processed into an oxide semiconductorlayer 453 formed of a stack of island-shaped oxide semiconductor layers(see FIG. 16C). In the drawing, the interface between the firstcrystalline oxide semiconductor layer 450 a and the second crystallineoxide semiconductor layer 450 b is indicated by a dotted line fordescription of the stack of oxide semiconductor layers. However, adefinite interface does not exist. The interface is illustrated for easydescription.

The stack of the oxide semiconductor layers can be processed by beingetched after a mask having a desired shape is formed over the stack ofthe oxide semiconductor layers. The mask can be formed by a method suchas photolithography. Alternatively, the mask may be formed by a methodsuch as an ink-jet method.

For the etching of the stack of the oxide semiconductor layers, eitherdry etching or wet etching may be employed. Needless to say, both ofthem may be employed in combination.

A feature of the first crystalline oxide semiconductor layer and thesecond crystalline oxide semiconductor layer obtained by the aboveformation method is that they have c-axis alignment. Note that the firstcrystalline oxide semiconductor layer and the second crystalline oxidesemiconductor comprise an oxide including a crystal with c-axisalignment (also referred to as C-Axis Aligned Crystal (CAAC)), which hasneither a single crystal structure nor an amorphous structure. The firstcrystalline oxide semiconductor layer and the second crystalline oxidesemiconductor layer partly include a crystal grain boundary.

Note that each of the first and second crystalline oxide semiconductorlayers can be formed using an oxide semiconductor described in any ofthe above embodiments.

Without limitation to the two-layer structure in which the secondcrystalline oxide semiconductor layer is formed over the firstcrystalline oxide semiconductor layer, a stacked structure includingthree or more layers may be formed by repeatedly performing a process offilm formation and heat treatment for forming a third crystalline oxidesemiconductor layer after the second crystalline oxide semiconductorlayer is formed.

The oxide semiconductor layer 453 formed of the stack of oxidesemiconductor layers formed by the above method can be used as the oxidesemiconductor layer 644 illustrated in FIG. 6 to FIG. 9.

In the transistor including the stack of oxide semiconductor layer ofthis embodiment as the oxide semiconductor layer 644, current mainlyflows along the interface of the stack of the oxide semiconductorlayers; therefore, even when the transistor is irradiated with light oreven when a BT stress is applied to the transistor, deterioration oftransistor characteristics is suppressed or reduced.

By forming a transistor with use of a stack of a first crystalline oxidesemiconductor layer and a second crystalline oxide semiconductor layer,like the oxide semiconductor layer 453, the transistor can have stableelectric characteristics and high reliability.

This embodiment can be combined as appropriate with any of the otherembodiments.

Embodiment 8

In this embodiment, an oxide including a crystal with c-axis alignment(also referred to as C-Axis Aligned Crystal (CAAC)), which has atriangular or hexagonal atomic arrangement when seen from the directionof an a-b plane, a surface, or an interface will be described. In thecrystal, metal atoms are arranged in a layered manner, or metal atomsand oxygen atoms are arranged in a layered manner along the c-axis, andthe directions of the a-axis or the b-axis varies in the a-b plane (thecrystal rotates on the c-axis).

In a broad sense, an oxide including CAAC means a non-single crystaloxide including a phase which has a triangular, hexagonal, regulartriangular, or regular hexagonal atomic arrangement when seen from thedirection perpendicular to the a-b plane and in which metal atoms arearranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis direction.

The CAAC is not a single crystal, but this does not mean that the CAACis composed of only an amorphous component. Although the CAAC includes acrystallized portion (crystalline portion), a boundary between onecrystalline portion and another crystalline portion is not clear in somecases.

In the case where oxygen is included in the CAAC, nitrogen may besubstituted for part of oxygen included in the CAAC. The c-axes ofindividual crystalline portions included in the CAAC may be aligned inone direction (e.g., a direction perpendicular to a surface of asubstrate over which the CAAC is formed or a surface of the CAAC).Alternatively, the normals of the a-b planes of the individualcrystalline portions included in the CAAC may be aligned in onedirection (e.g., a direction perpendicular to a surface of a substrateover which the CAAC is formed or a surface of the CAAC).

The CAAC becomes a conductor, a semiconductor, or an insulator dependingon its composition or the like. The CAAC transmits or does not transmitvisible light depending on its composition or the like.

As an example of such a CAAC, there is a crystal which is formed into afilm shape and has a triangular or hexagonal atomic arrangement whenobserved from the direction perpendicular to a surface of the film or asurface of a supporting substrate, and in which metal atoms are arrangedin a layered manner or metal atoms and oxygen atoms (or nitrogen atoms)are arranged in a layered manner when a cross section of the film isobserved.

An example of a crystal structure of the CAAC will be described indetail with reference to FIGS. 19A to 19E, FIGS. 20A to 20C, and FIGS.21A to 21C. In FIGS. 19A to 19E, FIGS. 20A to 20C, and FIGS. 21A to 21C,the vertical direction corresponds to the c-axis direction and a planeperpendicular to the c-axis direction corresponds to the a-b plane,unless otherwise specified. When the expressions “an upper half” and “alower half” are simply used, they refer to an upper half above the a-bplane and a lower half below the a-b plane (an upper half and a lowerhalf with respect to the a-b plane). Furthermore, in FIGS. 19A to 19E, Osurrounded by a circle represents tetracoordinate O and O surrounded bya double circle represents tricoordinate O.

FIG. 19A illustrates a structure including one hexacoordinate In atomand six tetracoordinate oxygen (hereinafter referred to astetracoordinate O) atoms proximate to the In atom. Here, a structureincluding one metal atom and oxygen atoms proximate thereto is referredto as a small group. The structure in FIG. 19A is actually an octahedralstructure, but is illustrated as a planar structure for simplicity. Notethat three tetracoordinate O atoms exist in each of an upper half and alower half in FIG. 19A. In the small group illustrated in FIG. 19A,electric charge is 0.

FIG. 19B illustrates a structure including one pentacoordinate Ga atom,three tricoordinate oxygen (hereinafter referred to as tricoordinate O)atoms proximate to the Ga atom, and two tetracoordinate O atomsproximate to the Ga atom. All the tricoordinate O atoms exist on the a-bplane. One of tetracoordinate O atom exists in an upper half and theother tetracoordinate O atom exists in a lower half in FIG. 19B. Thestructure illustrated in FIG. 19B can be employed using an In atombecause an In atom can have five ligands. In the small group illustratedin FIG. 19B, electric charge is 0.

FIG. 19C illustrates a structure including one tetracoordinate Zn atomand four tetracoordinate O atoms proximate to the Zn atom. In FIG. 19C,one tetracoordinate O atom exists in an upper half and threetetracoordinate O atoms exist in a lower half. Alternatively, threetetracoordinate O atoms may exist in the upper half and onetetracoordinate O atom may exist in the lower half in FIG. 19C. In thesmall group illustrated in FIG. 19C, electric charge is O.

FIG. 19D illustrates a structure including one hexacoordinate Sn atomand six tetracoordinate O atoms proximate to the Sn atom. In FIG. 19D,three tetracoordinate O atoms exist in an upper half and the other threetetracoordinate O atoms exist in a lower half. In the small groupillustrated in FIG. 19D, electric charge is +1.

FIG. 19E illustrates a small group including two Zn atoms. In FIG. 19E,one tetracoordinate O atom exists in each of an upper half and a lowerhalf. In the small group illustrated in FIG. 19E, electric charge is −1.

Here, a plurality of small groups form a medium group, and a pluralityof medium groups form a large group (also referred to as a unit cell).

Now, a rule of bonding between the small groups will be described. Thethree O atoms in the upper half with respect to the hexacoordinate Inatom in FIG. 19A each have three proximate In atoms in the downwarddirection, and the three O atoms in the lower half each have threeproximate In atoms in the upward direction. The one O atom in the upperhalf with respect to the pentacoordinate Ga atom has one proximate Gaatom in the downward direction, and the one O atom in the lower half hasone proximate Ga atom in the upward direction. The one O atom in theupper half with respect to the tetracoordinate Zn atom has one proximateZn atom in the downward direction, and the three O atoms in the lowerhalf each have three proximate Zn atoms in the upward direction. In thismanner, the number of the tetracoordinate O atoms above the metal atomis equal to the number of the metal atoms proximate to and below each ofthe tetracoordinate O atoms. Similarly, the number of thetetracoordinate O atoms below the metal atom is equal to the number ofthe metal atoms proximate to and above each of the tetracoordinate Oatoms. Since the coordination number of the tetracoordinate O atom is 4,the sum of the number of the metal atoms proximate to and below the Oatom and the number of the metal atoms proximate to and above the O atomis 4. Accordingly, when the sum of the number of tetracoordinate O atomsabove a metal atom and the number of tetracoordinate O atoms belowanother metal atom is 4, the two kinds of small groups including themetal atoms can be bonded. For example, in the case where thehexacoordinate metal (In or Sn) atom is bonded through threetetracoordinate O atoms in the lower half, it is bonded to thepentacoordinate metal (Ga or In) atom or the tetracoordinate metal (Zn)atom.

A metal atom whose coordination number is 4, 5, or 6 is bonded toanother metal atom through a tetracoordinate O atom in the c-axisdirection. In addition to the above, a medium group can be formed in adifferent manner by combining a plurality of small groups so that thetotal electric charge of the layered structure is 0.

FIG. 20A illustrates a model of a medium group included in a layeredstructure of an In—Sn—Zn—O-based material. FIG. 20B illustrates a largegroup including three medium groups. Note that FIG. 20C illustrates anatomic arrangement in the case where the layered structure in FIG. 20Bis observed from the c-axis direction.

In FIG. 20A, a tricoordinate O atom is omitted for simplicity, and atetracoordinate O atom is illustrated by a circle; the number in thecircle shows the number of tetracoordinate O atoms. For example, threetetracoordinate O atoms existing in each of an upper half and a lowerhalf with respect to a Sn atom are denoted by circled 3. Similarly, inFIG. 20A, one tetracoordinate O atom existing in each of an upper halfand a lower half with respect to an In atom is denoted by circled 1.FIG. 20A also illustrates a Zn atom proximate to one tetracoordinate Oatom in a lower half and three tetracoordinate O atoms in an upper half,and a Zn atom proximate to one tetracoordinate O atom in an upper halfand three tetracoordinate O atoms in a lower half.

In the medium group included in the layered structure of theIn—Sn—Zn—O-based material in FIG. 20A, in the order starting from thetop, a Sn atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to an In atom proximate to onetetracoordinate O atom in each of an upper half and a lower half, the Inatom is bonded to a Zn atom proximate to three tetracoordinate O atomsin an upper half, the Zn atom is bonded to an In atom proximate to threetetracoordinate O atoms in each of an upper half and a lower halfthrough one tetracoordinate O atom in a lower half with respect to theZn atom, the In atom is bonded to a small group that includes two Znatoms and is proximate to one tetracoordinate O atom in an upper half,and the small group is bonded to a Sn atom proximate to threetetracoordinate O atoms in each of an upper half and a lower halfthrough one tetracoordinate O atom in a lower half with respect to thesmall group. A plurality of such medium groups is bonded, so that alarge group is formed.

Here, electric charge for one bond of a tricoordinate O atom andelectric charge for one bond of a tetracoordinate O atom can be assumedto be −0.667 and −0.5, respectively. For example, electric charge of a(hexacoordinate or pentacoordinate) In atom, electric charge of a(tetracoordinate) Zn atom, and electric charge of a (pentacoordinate orhexacoordinate) Sn atom are +3, +2, and +4, respectively. Accordingly,electric charge of a small group including a Sn atom is +1. Therefore,electric charge of −1, which cancels +1, is needed to form a layeredstructure including a Sn atom. As a structure having electric charge of−1, the small group including two Zn atoms as illustrated in FIG. 19Ecan be given. For example, with one small group including two Zn atoms,electric charge of one small group including a Sn atom can be cancelled,so that the total electric charge of the layered structure can be 0.

When the large group illustrated in FIG. 20B is repeated, anIn—Sn—Zn—O-based crystal (In₂SnZn₃O₈) can be obtained. Note that alayered structure of the obtained In—Sn—Zn—O-based crystal can beexpressed as a composition formula, In₂SnZn₂O₇(ZnO)_(m) (m is 0 or anatural number).

The above-described rule also applies to the following materials: afour-component metal oxide such as an In—Sn—Ga—Zn-based oxide material;a three-component metal oxide such as an In—Ga—Zn-based oxide material(also referred to as IGZO), an In—Al—Zn-based oxide material, aSn—Ga—Zn-based oxide material, an Al—Ga—Zn-based oxide material, aSn—Al—Zn-based oxide material, an In—Hf—Zn-based oxide material, anIn—La—Zn-based oxide material, an In—Ce—Zn-based oxide material, anIn—Pr—Zn-based oxide material, an In—Nd—Zn-based oxide material, anIn—Pm—Zn-based oxide material, an In—Sm—Zn-based oxide material, anIn—Eu—Zn-based oxide material, an In—Gd—Zn-based oxide material, anIn—Tb—Zn-based oxide material, an In—Dy—Zn-based oxide material, anIn—Ho—Zn-based oxide material, an In—Er—Zn-based oxide material, anIn—Tm—Zn-based oxide material, an In—Yb—Zn-based oxide material, or anIn—Lu—Zn-based oxide material; a two-component metal oxide such as anIn—Zn-based oxide material, a Sn—Zn-based oxide material, an Al—Zn-basedoxide material, a Zn—Mg-based oxide material, a Sn—Mg-based oxidematerial, an In—Mg-based oxide material, or an In—Ga-based oxidematerial; and the like.

As an example, FIG. 21A illustrates a model of a medium group includedin a layered structure of an In—Ga—Zn—O-based material.

In the medium group included in the layered structure of theIn—Ga—Zn—O-based material in FIG. 21A, in the order starting from thetop, an In atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to a Zn atom proximate to onetetracoordinate O atom in an upper half, the Zn atom is bonded to a Gaatom proximate to one tetracoordinate O atom in each of an upper halfand a lower half through three tetracoordinate O atoms in a lower halfwith respect to the Zn atom, and the Ga atom is bonded to an In atomproximate to three tetracoordinate O atoms in each of an upper half anda lower half through one tetracoordinate O atom in a lower half withrespect to the Ga atom. A plurality of such medium groups is bonded, sothat a large group is formed.

FIG. 21B illustrates a large group including three medium groups. Notethat FIG. 21C illustrates an atomic arrangement in the case where thelayered structure in FIG. 21B is observed from the c-axis direction.

Here, since electric charge of a (hexacoordinate or pentacoordinate) Inatom, electric charge of a (tetracoordinate) Zn atom, and electriccharge of a (pentacoordinate) Ga atom are +3, +2, and +3, respectively,electric charge of a small group including any of an In atom, a Zn atom,and a Ga atom is 0. As a result, the total electric charge of a mediumgroup having a combination of such small groups is always 0.

In order to form the layered structure of the In—Ga—Zn—O-based material,a large group can be formed using not only the medium group illustratedin FIG. 21A but also a medium group in which the arrangement of the Inatom, the Ga atom, and the Zn atom is different from that in FIG. 21A.

This embodiment can be combined as appropriate with any of the otherembodiments.

Embodiment 9

The actually measured field-effect mobility of an insulated gatetransistor can be lower than its original mobility because of a varietyof reasons; this phenomenon occurs not only in the case of using anoxide semiconductor. One of the reasons that reduce the mobility is adefect inside a semiconductor or a defect at an interface between thesemiconductor and an insulating film. When a Levinson model is used, thefield-effect mobility on the assumption that no defect exists inside thesemiconductor can be calculated theoretically.

Assuming that the original mobility and the measured field-effectmobility of a semiconductor are μ₀ and μ, respectively, and a potentialbarrier (such as a grain boundary) exists in the semiconductor, themeasured field-effect mobility can be expressed as the followingformula.

$\begin{matrix}{\mu = {\mu_{0}{\exp\left( {- \frac{E}{kT}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Here, E represents the height of the potential barrier, k represents theBoltzmann constant, and T represents the absolute temperature. When thepotential barrier is assumed to be attributed to a defect, the height ofthe potential barrier can be expressed as the following formulaaccording to the Levinson model.

$\begin{matrix}{E = {\frac{e^{2}N^{2}}{8\; ɛ\; n} = \frac{e^{3}N^{2}}{8\; ɛ\; C_{ox}\; V_{g}}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Here, e represents the elementary charge, N represents the averagedefect density per unit area in a channel, ∈ represents the dielectricconstant of the semiconductor, n represents the number of carriers perunit area in the channel, C_(ox) represents the capacitance per unitarea, V_(g) represents the gate voltage, and t represents the thicknessof the channel. In the case where the thickness of the semiconductorlayer is less than or equal to 30 nm, the thickness of the channel maybe regarded as being the same as the thickness of the semiconductorlayer. The drain current I_(d) in a linear region can be expressed asthe following formula.

$\begin{matrix}{I_{d} = {\frac{W_{\mu}\; V_{g}V_{d}C_{ox}}{L}{\exp\left( {- \frac{E}{kT}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Here, L represents the channel length and W represents the channelwidth, and L and W are each 10 μm. In addition, V_(d) represents thedrain voltage. When dividing both sides of the above equation by V_(g)and then taking logarithms of both sides, the following formula can beobtained.

$\begin{matrix}{{\ln\left( \frac{I_{d}}{V_{g}} \right)} = {{{\ln\left( \frac{W_{\mu}V_{d}C_{ox}}{L} \right)} - \frac{E}{kT}} = {{\ln\left( \frac{W_{\mu}V_{d}C_{ox}}{L} \right)} - \frac{e^{3}N^{2}t}{8\;{kT}\; ɛ\; C_{ox}V_{g}}}}} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack\end{matrix}$

The right side of Formula 5 is a function of V_(g). From the formula, itis found that the defect density N can be obtained from the slope of aline in a graph which is obtained by plotting actual measured valueswith ln(I_(d)/V_(g)) as the ordinate and 1/V_(g) as the abscissa. Thatis, the defect density can be evaluated from the I_(d)−V_(g)characteristics of the transistor. The defect density N of an oxidesemiconductor in which the ratio of indium (In), tin (Sn), and zinc (Zn)is 1:1:1 is approximately 1×10¹²/cm².

On the basis of the defect density obtained in this manner, or the like,μ₀ can be calculated to be 120 cm²/Vs from Formula 2 and Formula 3. Themeasured mobility of an In—Sn—Zn oxide including a defect isapproximately 35 cm²/Vs. However, assuming that no defect exists insidethe semiconductor and at the interface between the semiconductor and aninsulating film, the mobility μ₀ of the oxide semiconductor is expectedto be 120 cm²/Vs.

Note that even when no defect exists inside a semiconductor, scatteringat an interface between a channel and a gate insulating layer affectsthe transport property of the transistor. In other words, the mobilityμ₁ at a position that is distance x away from the interface between thechannel and the gate insulating layer can be expressed as the followingformula.

$\begin{matrix}{\frac{1}{\mu_{1}} = {\frac{1}{\mu_{0}} + {\frac{D}{B}{\exp\left( {- \frac{x}{G}} \right)}}}} & \left\lbrack {{Formula}\mspace{14mu} 6} \right\rbrack\end{matrix}$

Here, D represents the electric field in the gate direction, and B and Gare constants. B and G can be obtained from actual measurement results;according to the above measurement results, B is 4.75×10⁷ cm/s and G is10 nm (the depth to which the influence of interface scatteringreaches). When D is increased (i.e., when the gate voltage isincreased), the second term of Formula 6 is increased and accordinglythe mobility μ₁ is decreased.

Calculation results of the mobility μ₂ of a transistor whose channelincludes an ideal oxide semiconductor without a defect inside thesemiconductor are shown in FIG. 22. For the calculation, devicesimulation software Sentaurus Device manufactured by Synopsys, Inc. wasused, and the bandgap, the electron affinity, the relative permittivity,and the thickness of the oxide semiconductor were assumed to be 2.8 eV,4.7 eV, 15, and 15 nm, respectively. These values were obtained bymeasurement of a thin film that was formed by a sputtering method.

Further, the work functions of a gate, a source, and a drain wereassumed to be 5.5 eV, 4.6 eV, and 4.6 eV, respectively. The thickness ofa gate insulating layer was assumed to be 100 nm, and the relativepermittivity thereof was assumed to be 4.1. The channel length and thechannel width were each assumed to be 10 μm, and the drain voltage V_(d)was assumed to be 0.1 V.

As shown in FIG. 22, the mobility has a peak of 100 cm²/Vs or more at agate voltage that is a little over 1 V and is decreased as the gatevoltage becomes higher because the influence of interface scattering isincreased. Note that in order to reduce interface scattering, it isdesirable that a surface of the semiconductor layer be flat at theatomic level (atomic layer flatness).

Calculation results of characteristics of minute transistors which aremanufactured using an oxide semiconductor having such a mobility areshown in FIGS. 23A to 23C, FIGS. 24A to 24C, and FIGS. 25A to 25C. FIGS.26A and 26B illustrate cross-sectional structures of the transistorsused for the calculation. The transistors illustrated in FIGS. 26A and26B each include a semiconductor region 8103 a and a semiconductorregion 8103 c which have n⁺-type conductivity in an oxide semiconductorlayer. The resistivities of the semiconductor region 8103 a and thesemiconductor region 8103 c are 2×10⁻³ Ωcm.

The transistor illustrated in FIG. 26A is formed over a base insulatinglayer 8101 and an embedded insulator 8102 which is embedded in the baseinsulating layer 8101 and formed of aluminum oxide. The transistorincludes the semiconductor region 8103 a, the semiconductor region 8103c, an intrinsic semiconductor region 8103 b serving as a channelformation region therebetween, and a gate electrode 8105. The width ofthe gate electrode 8105 is 33 nm.

A gate insulating layer 8104 is formed between the gate electrode 8105and the semiconductor region 8103 b. In addition, a sidewall insulator8106 a and a sidewall insulator 8106 b are formed on both side surfacesof the gate electrode 8105, and an insulator 8107 is formed over thegate electrode 8105 so as to prevent a short circuit between the gateelectrode 8105 and another wiring. The sidewall insulator has a width of5 nm. A source 8108 a and a drain 8108 b are provided in contact withthe semiconductor region 8103 a and the semiconductor region 8103 c,respectively. Note that the channel width of this transistor is 40 nm.

The transistor of FIG. 26B is the same as the transistor of FIG. 26A inthat it is formed over the base insulating layer 8101 and the embeddedinsulator 8102 formed of aluminum oxide and that it includes thesemiconductor region 8103 a, the semiconductor region 8103 c, theintrinsic semiconductor region 8103 b provided therebetween, the gateelectrode 8105 having a width of 33 nm, the gate insulating layer 8104,the sidewall insulator 8106 a, the sidewall insulator 8106 b, theinsulator 8107, the source 8108 a, and the drain 8108 b.

The transistor illustrated in FIG. 26A is different from the transistorillustrated in FIG. 26B in the conductivity type of semiconductorregions under the sidewall insulator 8106 a and the sidewall insulator8106 b. In the transistor illustrated in FIG. 26A, the semiconductorregions under the sidewall insulator 8106 a and the sidewall insulator8106 b are part of the semiconductor region 8103 a and the semiconductorregion 8103 c having n⁺-type conductivity, whereas in the transistorillustrated in FIG. 26B, the semiconductor regions under the sidewallinsulator 8106 a and the sidewall insulator 8106 b are part of theintrinsic semiconductor region 8103 b. In other words, in thesemiconductor layer of FIG. 26B, a region having a width of L_(off)which overlaps with neither the semiconductor region 8103 a (thesemiconductor region 8103 c) nor the gate electrode 8105 is provided.This region is called an offset region, and the width L_(off) is calledan offset length. As is seen from the drawing, the offset length isequal to the width of the sidewall insulator 8106 a (the sidewallinsulator 8106 b).

The other parameters used in calculation are as described above. For thecalculation, device simulation software Sentaurus Device manufactured bySynopsys, Inc. was used. FIGS. 23A to 23C show the gate voltage (V_(g):a potential difference between the gate and the source) dependence ofthe drain current (I_(d), a solid line) and the mobility (μ, a dottedline) of the transistor having the structure illustrated in FIG. 26A.The drain current I_(d) is obtained by calculation under the assumptionthat the drain voltage (a potential difference between the drain and thesource) is +1 V and the mobility μ is obtained by calculation under theassumption that the drain voltage is +0.1 V.

FIG. 23A shows the transistor characteristics in the case where thethickness of the gate insulating layer is 15 nm, FIG. 23B shows thetransistor characteristics in the case where the thickness of the gateinsulating layer is 10 nm, and FIG. 23C shows the transistorcharacteristics in the case where the thickness of the gate insulatinglayer is 5 nm. As the gate insulating layer is thinner, the draincurrent I_(d) (off-state current) particularly in an off state issignificantly decreased. In contrast, there is no noticeable change inthe peak value of the mobility μ and the drain current μ_(d) (on-statecurrent) in an on state. The graphs show that the drain current exceeds10 μA, which is needed in a storage element and the like, at a gatevoltage of around 1 V.

FIGS. 24A to 24C show the gate voltage V_(g) dependence of the draincurrent I_(d) (a solid line) and the mobility μ (a dotted line) of thetransistor having the structure illustrated in FIG. 26B where the offsetlength L_(off) is 5 nm. The drain current I_(d) is obtained bycalculation under the assumption that the drain voltage is +1 V and themobility μ is obtained by calculation under the assumption that thedrain voltage is +0.1 V. FIG. 24A shows the transistor characteristicsin the case where the thickness of the gate insulating layer is 15 nm,FIG. 24B shows the transistor characteristics in the case where thethickness of the gate insulating layer is 10 nm, and FIG. 24C shows thetransistor characteristics in the case where the thickness of the gateinsulating layer is 5 nm.

Further, FIGS. 25A to 25C show the gate voltage dependence of the draincurrent I_(d) (a solid line) and the mobility μ (a dotted line) of thetransistor having the structure illustrated in FIG. 26B where the offsetlength L_(off) is 15 nm. The drain current I_(d) is obtained bycalculation under the assumption that the drain voltage is +1 V and themobility μ is obtained by calculation under the assumption that thedrain voltage is +0.1 V. FIG. 25A shows the transistor characteristicsin the case where the thickness of the gate insulating layer is 15 nm,FIG. 25B shows the transistor characteristics in the case where thethickness of the gate insulating layer is 10 nm, and FIG. 25C shows thetransistor characteristics in the case where the thickness of the gateinsulating layer is 5 nm.

In either of the structures, as the gate insulating layer is thinner,the off-state current is significantly decreased, whereas no noticeablechange arises in the peak value of the mobility and the on-statecurrent.

Note that the peak of the mobility μ is approximately 80 cm²/Vs in FIGS.23A to 23C, approximately 60 cm²/Vs in FIGS. 24A to 24C, andapproximately 40 cm²/Vs in FIGS. 25A to 25C; thus, the peak of themobility μ is decreased as the offset length L_(off) is increased.Further, the same applies to the off-state current. The on-state currentis also decreased as the offset length L_(off) is increased; however,the decrease in the on-state current is much more gradual than thedecrease in the off-state current. Further, the graphs show that ineither of the structures, the drain current exceeds 10 μA, which isneeded in a storage element and the like, at a gate voltage of around 1V.

This embodiment can be combined as appropriate with any of the otherembodiments.

Embodiment 10

In this embodiment, as an example of a transistor in which a channel isformed in an oxide semiconductor layer, a transistor in which an oxidesemiconductor containing In, Sn, and Zn as main components is used as anoxide semiconductor will be described in detail. A transistor in whichan oxide semiconductor containing In, Sn, and Zn as main components isused as a channel formation region can have favorable characteristics bydepositing the oxide semiconductor while heating a substrate or byperforming heat treatment after an oxide semiconductor film is formed.Note that a main component refers to an element included in acomposition at 5 atomic % or more.

By intentionally heating the substrate after formation of the oxidesemiconductor film containing In, Sn, and Zn as main components, thefield-effect mobility of the transistor can be improved. Further, thethreshold voltage of the transistor can be positively shifted to makethe transistor normally off.

As an example, FIGS. 27A to 27C each show characteristics of atransistor in which an oxide semiconductor film containing In, Sn, andZn as main components and having a channel length L of 3 μm and achannel width W of 10 μm, and a gate insulating layer with a thicknessof 100 nm are used. Note that V_(d) was set to 10 V.

FIG. 27A shows characteristics of a transistor whose oxide semiconductorfilm containing In, Sn, and Zn as main components is formed by asputtering method without heating a substrate intentionally. Thefield-effect mobility obtained in this case is 18.8 cm²/Vsec. On theother hand, when the oxide semiconductor film containing In, Sn, and Znas main components is formed while heating the substrate intentionally,the field-effect mobility can be improved. FIG. 27B showscharacteristics of a transistor whose oxide semiconductor filmcontaining In, Sn, and Zn as main components is formed while heating asubstrate at 200° C. The field-effect mobility in this case is 32.2cm²/Vsec.

The field-effect mobility can be further improved by performing heattreatment after formation of the oxide semiconductor film containing In,Sn, and Zn as main components. FIG. 27C shows characteristics of atransistor whose oxide semiconductor film containing In, Sn, and Zn asmain components is formed by sputtering at 200° C. and then subjected toheat treatment at 650° C. The field-effect mobility obtained in thiscase is 34.5 cm²/Vsec.

The intentional heating of the substrate is expected to have an effectof reducing moisture taken into the oxide semiconductor film during thefilm formation by sputtering. Further, the heat treatment after filmformation enables hydrogen, a hydroxyl group, or moisture to be releasedand removed from the oxide semiconductor film. In this manner, thefield-effect mobility can be improved. Such an improvement infield-effect mobility is presumed to be achieved not only by removal ofimpurities by dehydration or dehydrogenation but also by a reduction ininteratomic distance due to an increase in density. The oxidesemiconductor can be crystallized by being highly purified by removal ofimpurities from the oxide semiconductor. In the case of using such ahighly purified non-single-crystal oxide semiconductor, ideally, afield-effect mobility exceeding 100 cm²/Vsec is expected to be realized.

The oxide semiconductor containing In, Sn, and Zn as main components maybe crystallized in the following manner: oxygen ions are implanted intothe oxide semiconductor, hydrogen, a hydroxyl group, or moistureincluded in the oxide semiconductor is released by heat treatment, andthe oxide semiconductor is crystallized through the heat treatment or byanother heat treatment performed later. By such crystallizationtreatment or recrystallization treatment, a non-single-crystal oxidesemiconductor having favorable crystallinity can be obtained.

The intentional heating of the substrate during film formation and/orthe heat treatment after the film formation contributes not only toimproving field-effect mobility but also to making the transistornormally off. In a transistor in which an oxide semiconductor film thatcontains In, Sn, and Zn as main components and is formed without heatinga substrate intentionally is used as a channel formation region, thethreshold voltage tends to be shifted negatively. However, when theoxide semiconductor film formed while heating the substrateintentionally is used, the problem of the negative shift of thethreshold voltage can be solved. That is, the threshold voltage isshifted so that the transistor becomes normally off; this tendency canbe confirmed by comparison between FIGS. 27A and 27B.

Note that the threshold voltage can also be controlled by changing theratio of In, Sn, and Zn; when the composition ratio of In, Sn, and Zn is2:1:3, a normally-off transistor is expected to be formed. In addition,an oxide semiconductor film having high crystallinity can be obtained bysetting the composition ratio of a target as follows: In:Sn:Zn=2:1:3.

The temperature of the intentional heating of the substrate or thetemperature of the heat treatment is 150° C. or higher, preferably 200°C. or higher, further preferably 400° C. or higher. When film formationor heat treatment is performed at a high temperature, the transistor canbe normally off.

By intentionally heating the substrate during film formation and/or byperforming heat treatment after the film formation, the stabilityagainst a gate-bias stress can be increased. For example, when a gatebias is applied with an intensity of 2 MV/cm at 150° C. for one hour,drift of the threshold voltage can be less than ±1.5 V, preferably lessthan ±1.0 V.

A BT test was performed on the following two transistors: Sample 1 onwhich heat treatment was not performed after formation of an oxidesemiconductor film, and Sample 2 on which heat treatment at 650° C. wasperformed after formation of an oxide semiconductor film.

First, V_(g)−I_(d) characteristics of the transistors were measured at asubstrate temperature of 25° C. and V_(ds) of 10 V. Note that V_(ds)refers to a drain voltage (a potential difference between a drain and asource). Then, the substrate temperature was set to 150° C. and V_(ds)was set to 0.1 V. After that, 20 V of V_(g) was applied so that theintensity of an electric field applied to gate insulating layers was 2MV/cm, and the condition was kept for one hour. Next, V_(g) was set to 0V. Then, V_(g)−I_(d) characteristics of the transistors were measured ata substrate temperature of 25° C. and V_(ds) of 10 V. This process iscalled a positive BT test.

In a similar manner, first, V_(g)−I_(d) characteristics of thetransistors were measured at a substrate temperature of 25° C. andV_(ds) of 10 V. Then, the substrate temperature was set at 150° C. andV_(ds) was set to 0.1 V. After that, −20 V of V_(g) was applied so thatthe intensity of an electric field applied to the gate insulating layerswas −2 MV/cm, and the condition was kept for one hour. Next, V_(g) wasset to 0 V. Then, V_(g)−I_(d) characteristics of the transistors weremeasured at a substrate temperature of 25° C. and V_(ds) of 10 V. Thisprocess is called a negative BT test.

FIGS. 28A and 28B show a result of the positive BT test of Sample 1 anda result of the negative BT test of Sample 1, respectively. FIGS. 29Aand 29B show a result of the positive BT test of Sample 2 and a resultof the negative BT test of Sample 2, respectively.

The amount of shift in the threshold voltage of Sample 1 due to thepositive BT test and that due to the negative BT test were 1.80 V and−0.42 V, respectively. The amount of shift in the threshold voltage ofSample 2 due to the positive BT test and that due to the negative BTtest were 0.79 V and 0.76 V, respectively. It is found that, in each ofSample 1 and Sample 2, the amount of shift in the threshold voltagebetween before and after the BT tests is small and the reliabilitythereof is high.

The heat treatment can be performed in an oxygen atmosphere;alternatively, the heat treatment may be performed first in anatmosphere of nitrogen or an inert gas or under reduced pressure, andthen in an atmosphere including oxygen. Oxygen is supplied to the oxidesemiconductor after dehydration or dehydrogenation, whereby an effect ofthe heat treatment can be further increased. As a method for supplyingoxygen after dehydration or dehydrogenation, a method in which oxygenions are accelerated by an electric field and implanted into the oxidesemiconductor film may be employed.

A defect due to oxygen deficiency is easily caused in the oxidesemiconductor or at an interface between the oxide semiconductor and afilm in contact with the oxide semiconductor; however, when excessoxygen is included in the oxide semiconductor by the heat treatment,oxygen deficiency caused constantly can be compensated for with excessoxygen. The excess oxygen is oxygen existing mainly between lattices.When the concentration of excess oxygen is set to higher than or equalto 1×10¹⁶/cm³ and lower than or equal to 2×10²⁰/cm³, excess oxygen canbe included in the oxide semiconductor without causing crystaldistortion or the like.

When heat treatment is performed so that at least part of the oxidesemiconductor includes crystal, a more stable oxide semiconductor filmcan be obtained. For example, when an oxide semiconductor film which isformed by sputtering using a target in which a composition ratio of In,Sn, and Zn is 1:1:1, without heating a substrate intentionally, isanalyzed by X-ray diffraction (XRD), a halo pattern is observed. Theformed oxide semiconductor film can be crystallized by being subjectedto heat treatment. The temperature of the heat treatment can be set asappropriate; when the heat treatment is performed at 650° C., forexample, a clear diffraction peak can be observed in an X-raydiffraction analysis.

An XRD analysis of an In—Sn—Zn—O film was conducted. For the XRDanalysis, an X-ray diffractometer D8 ADVANCE manufactured by Bruker AXSwas used, and an out-of-plane method was employed.

Sample A and Sample B were prepared and the XRD analysis was performedthereon. A method for manufacturing Sample A and Sample B will bedescribed below.

An In—Sn—Zn—O film with a thickness of 100 nm was formed over a quartzsubstrate that had been subjected to dehydrogenation treatment.

The In—Sn—Zn—O film was formed with a sputtering apparatus with a powerof 100 W (DC) in an oxygen atmosphere. An In—Sn—Zn—O target having anatomic ratio: In:Sn:Zn=1:1:1 was used as a target. Note that thesubstrate heating temperature in film formation was set at 200° C. Asample manufactured in this manner was used as Sample A.

Next, a sample manufactured by a method similar to that of Sample A wassubjected to heat treatment at 650° C. As the heat treatment, heattreatment in a nitrogen atmosphere was first performed for one hour andheat treatment in an oxygen atmosphere was further performed for onehour without lowering the temperature. A sample manufactured in thismanner was used as Sample B.

FIG. 30 shows XRD spectra of Sample A and Sample B. No peak derived fromcrystal was observed in Sample A, whereas peaks derived from crystalwere observed when 20 was around 35 deg. and 37 deg. to 38 deg. inSample B.

As described above, by intentionally heating a substrate duringdeposition of an oxide semiconductor containing In, Sn, and Zn as maincomponents and/or by performing heat treatment after the deposition,characteristics of a transistor can be improved.

These substrate heating and heat treatment have an effect of preventinghydrogen and a hydroxyl group, which are unfavorable impurities for anoxide semiconductor, from being included in the film or an effect ofremoving hydrogen and a hydroxyl group from the film. That is, an oxidesemiconductor can be highly purified by removing hydrogen serving as adonor impurity from the oxide semiconductor, whereby a normally-offtransistor can be obtained. The high purification of an oxidesemiconductor enables the off-state current of the transistor to be 1aA/μm or lower. Here, the unit of the off-state current is used toindicate current per micrometer of a channel width.

FIG. 31 shows a relation between the off-state current of a transistorand the inverse of substrate temperature (absolute temperature) atmeasurement. Here, for simplicity, the horizontal axis represents avalue (1000/T) obtained by multiplying an inverse of substratetemperature at measurement by 1000.

Specifically, as shown in FIG. 31, the off-state current can be 1 aA/μm(1×10⁻¹⁸ A/μm) or lower, 100 zA/μm (1×10⁻¹⁹ A/μm) or lower, and 1 zA/μm(1×10⁻²¹ A/μm) or lower when the substrate temperature is 125° C., 85°C., and room temperature (27° C.), respectively. Preferably, theoff-state current can be 0.1 aA/μm (1×10⁻¹⁹ A/μm) or lower, 10 zA/μm(1×10⁻²° A/μm) or lower, and 0.1 zA/μm (1×10⁻²² A/mm) or lower at 125°C., 85° C., and room temperature, respectively. The above values ofoff-state currents are clearly much lower than that of the transistorusing Si as a semiconductor film.

Note that in order to prevent hydrogen and moisture from being includedin the oxide semiconductor film during formation thereof, it ispreferable to increase the purity of a sputtering gas by sufficientlysuppressing leakage from the outside of a deposition chamber anddegasification through an inner wall of the deposition chamber. Forexample, a gas with a dew point of −70° C. or lower is preferably usedas the sputtering gas in order to prevent moisture from being includedin the film. In addition, it is preferable to use a target which ishighly purified so as not to include impurities such as hydrogen andmoisture. Although it is possible to remove moisture from a film of anoxide semiconductor containing In, Sn, and Zn as main components by heattreatment, a film which does not contain moisture originally ispreferably formed because moisture is released from the oxidesemiconductor including In, Sn, and Zn as main components at a highertemperature than from an oxide semiconductor including In, Ga, and Zn asmain components.

The relation between the substrate temperature and electriccharacteristics of a transistor formed using Sample B, on which heattreatment at 650° C. was performed after formation of the oxidesemiconductor film, was evaluated.

The transistor used for the measurement had a channel length L of 3 μm,a channel width W of 10 μm, L_(ov) of 0 μm, and dW of 0 μm. Note thatV_(ds) was set to 10 V. Note that the substrate temperature was −40° C.,−25° C., 25° C., 75° C., 125° C., and 150° C. Here, in a transistor, thewidth of a portion where a gate electrode overlaps with one of a pair ofelectrodes is referred to as L_(ov), and the width of a portion of thepair of electrodes, which does not overlap with an oxide semiconductorfilm, is referred to as dW.

FIG. 32 shows the V_(g) dependence of I_(d) (a solid line) andfield-effect mobility (a dotted line). FIG. 33A shows a relation betweenthe substrate temperature and the threshold voltage, and FIG. 33B showsa relation between the substrate temperature and the field-effectmobility.

From FIG. 33A, it is found that the threshold voltage gets lower as thesubstrate temperature increases. Note that the threshold voltage isdecreased from 1.09 V to −0.23 V in the range from −40° C. to 150° C.

From FIG. 33B, it is found that the field-effect mobility gets lower asthe substrate temperature increases. Note that the field-effect mobilityis decreased from 36 cm²/Vs to 32 cm²/VS in the range from −40° C. to150° C. Thus, it is found that variation in electric characteristics issmall in the above temperature range.

In a transistor in which such an oxide semiconductor containing In, Sn,and Zn as main components is used as a channel formation region, afield-effect mobility of 30 cm²/Vsec or higher, preferably 40 cm²/Vsecor higher, further preferably 60 cm²/Vsec or higher can be obtained withthe off-state current maintained at 1 aA/μm or lower, which can achieveon-state current needed for an LSI. For example, in an FET where L/W is33 nm/40 nm, an on-state current of 12 μA or higher can flow when thegate voltage is 2.7 V and the drain voltage is 1.0 V. In addition,sufficient electric characteristics can be ensured in a temperaturerange needed for operation of a transistor. With such characteristics,an integrated circuit having a novel function can be realized withoutdecreasing the operation speed even when a transistor including an oxidesemiconductor is also provided in an integrated circuit formed using aSi semiconductor.

This embodiment can be combined as appropriate with any of theabove-described embodiments.

Example 1

In this example, an example of a transistor including an In—Sn—Zn—O filmas an oxide semiconductor layer in which a channel is formed will bedescribed with reference to FIGS. 34A and 34B and the like.

FIGS. 34A and 34B are a top view and a cross-sectional view of acoplanar transistor having a top-gate top-contact structure. FIG. 34A isthe top view of the transistor. FIG. 34B illustrates cross section A1-A2along dashed-dotted line A-B in FIG. 34A.

The transistor illustrated in FIG. 34B includes a substrate 8500; a baseinsulating layer 8502 provided over the substrate 8500; a protectiveinsulating film 8504 provided in the periphery of the base insulatinglayer 8502; an oxide semiconductor layer 8506 provided over the baseinsulating layer 8502 and the protective insulating film 8504 andincluding a high-resistance region 8506 a and low-resistance regions8506 b; a gate insulating layer 8508 provided over the oxidesemiconductor layer 8506; a gate electrode 8510 provided to overlap withthe oxide semiconductor layer 8506 with the gate insulating layer 8508positioned therebetween; a sidewall insulating film 8512 provided incontact with a side surface of the gate electrode 8510; a pair ofelectrodes 8514 provided in contact with at least the low-resistanceregions 8506 b; an interlayer insulating film 8516 provided to cover atleast the oxide semiconductor layer 8506, the gate electrode 8510, andthe pair of electrodes 8514; and a wiring 8518 provided to be connectedto at least one of the pair of electrodes 8514 through an opening formedin the interlayer insulating film 8516.

Although not illustrated, a protective film may be provided to cover theinterlayer insulating film 8516 and the wiring 8518. With the protectivefilm, a minute amount of leakage current generated by surface conductionof the interlayer insulating film 8516 can be reduced and thus theoff-state current of the transistor can be reduced.

This example can be combined as appropriate with any of theabove-described embodiments.

Example 2

In this embodiment, another example of a transistor in which anIn—Sn—Zn—O film, which is different from that of the above example, isused as an oxide semiconductor layer will be described with reference toFIGS. 35A and 35B.

FIGS. 35A and 35B are a top view and a cross-sectional view whichillustrates a structure of a transistor manufactured in this example.FIG. 35A is the top view of the transistor. FIG. 35B is across-sectional view along dashed-dotted line B1-B2 in FIG. 35A.

The transistor illustrated in FIG. 35B includes a substrate 8600; a baseinsulating layer 8602 provided over the substrate 8600; an oxidesemiconductor layer 8606 provided over the base insulating layer 8602; apair of electrodes 8614 in contact with the oxide semiconductor layer8606; a gate insulating layer 8608 provided over the oxide semiconductorlayer 8606 and the pair of electrodes 8614; a gate electrode 8610provided to overlap with the oxide semiconductor layer 8606 with thegate insulating layer 8608 positioned therebetween; an interlayerinsulating film 8616 provided to cover the gate insulating layer 8608and the gate electrode 8610; wirings 8618 connected to the pair ofelectrodes 8614 through openings formed in the interlayer insulatingfilm 8616; and a protective film 8620 provided to cover the interlayerinsulating film 8616 and the wirings 8618.

As the substrate 8600, a glass substrate can be used. As the baseinsulating layer 8602, a silicon oxide film can be used. As the oxidesemiconductor layer 8606, an In—Sn—Zn—O film can be used. As the pair ofelectrodes 8614, a tungsten film can be used. As the gate insulatinglayer 8608, a silicon oxide film can be used. The gate electrode 8610can have a stacked structure of a tantalum nitride film and a tungstenfilm. The interlayer insulating film 8616 can have a stacked structureof a silicon oxynitride film and a polyimide film. The wirings 8618 caneach have a stacked structure in which a titanium film, an aluminumfilm, and a titanium film are formed in this order. As the protectivefilm 8620, a polyimide film can be used.

Note that in the transistor having the structure illustrated in FIG.35A, the width of a portion where the gate electrode 8610 overlaps withone of the pair of electrodes 8614 is referred to as L_(ov). Similarly,the width of a portion of the pair of electrodes 8614, which does notoverlap with the oxide semiconductor layer 8606, is referred to as dW.

This example can be combined as appropriate with any of theabove-described embodiments.

Example 3

With use of a signal processing circuit according to one embodiment ofthe present invention, a highly reliable electronic device and anelectronic device with low power consumption can be provided. Inparticular, when to a portable electronic device which has difficulty incontinuously receiving power, a signal processing circuit with low powerconsumption according to one embodiment of the present invention isadded as a component of the device, an advantage in increasing thecontinuous operation time can be obtained.

The signal processing circuit according to one embodiment of the presentinvention can be used for display devices, personal computers, or imagereproducing devices provided with recording media (typically, deviceswhich reproduce the content of recording media such as digital versatilediscs (DVDs) and have displays for displaying the reproduced images).Other than the above, as an electronic device which can employ thesignal processing circuit according to one embodiment of the presentinvention, mobile phones, portable game machines, portable informationterminals, e-book readers, cameras such as video cameras and digitalstill cameras, goggle-type displays (head mounted displays), navigationsystems, audio reproducing devices (e.g., car audio systems and digitalaudio players), copiers, facsimiles, printers, multifunction printers,automated teller machines (ATM), vending machines, and the like can begiven. Specific examples of these electronic devices are shown in FIGS.18A to 18F.

FIG. 18A illustrates an e-book reader including a housing 7001, adisplay portion 7002, and the like. The signal processing circuitaccording to one embodiment of the present invention can be used for anintegrated circuit used for controlling driving of the e-book reader.With use of the signal processing circuit according to one embodiment ofthe present invention for the integrated circuit for controlling drivingof the e-book reader, the e-book reader can reduce power consumption.When a flexible substrate is used, the signal processing circuit canhave flexibility, whereby a user-friendly e-book reader which isflexible and lightweight can be provided.

FIG. 18B illustrates a display device including a housing 7011, adisplay portion 7012, a supporting base 7013, and the like. The signalprocessing circuit according to one embodiment of the present inventioncan be used for an integrated circuit used for controlling driving ofthe display device. With use of the signal processing circuit accordingto one embodiment of the present invention for the integrated circuitfor controlling driving of the display device, the display device canreduce power consumption. The display device includes in its category,any information display device for personal computers, TV broadcastreception, advertisement, and the like.

FIG. 18C illustrates a display device including a housing 7021, adisplay portion 7022, and the like. The signal processing circuitaccording to one embodiment of the present invention can be used for anintegrated circuit used for controlling driving of the display device.With use of the signal processing circuit according to one embodiment ofthe present invention for the integrated circuit for controlling drivingof the display device, the display device can reduce power consumption.Moreover, with use of a flexible substrate, the signal processingcircuit can have flexibility. Thus, a user-friendly display device whichis flexible and lightweight can be provided. Accordingly, as illustratedin FIG. 18C, such a display device can be used while being fixed tofabric or the like, and an application range of the display device isdramatically widened.

FIG. 18D illustrates a portable game machine including a housing 7031, ahousing 7032, a display portion 7033, a display portion 7034, amicrophone 7035, speakers 7036, operation keys 7037, a stylus 7038, andthe like. The signal processing circuit according to one embodiment ofthe present invention can be used for an integrated circuit used forcontrolling driving of the portable game machine. With use of the signalprocessing circuit according to one embodiment of the present inventionfor the integrated circuit for controlling driving of the portable gamemachine, the portable game machine can reduce power consumption.Although the portable game machine illustrated in FIG. 18D has the twodisplay portions 7033 and 7034, the number of display portions includedin the portable game machines is not limited thereto.

FIG. 18E illustrates a mobile phone including a housing 7041, a displayportion 7042, an audio-input portion 7043, an audio-output portion 7044,operation keys 7045, a light-receiving portion 7046, and the like. Lightreceived in the light-receiving portion 7046 is converted intoelectrical signals, whereby external images can be loaded. The signalprocessing circuit according to one embodiment of the present inventioncan be used for an integrated circuit used for controlling driving ofthe mobile phone. With use of the signal processing circuit according toone embodiment of the present invention for the integrated circuit forcontrolling driving of the mobile phone, the mobile phone can reducepower consumption.

FIG. 18F illustrates a portable information terminal including a housing7051, a display portion 7052, operation keys 7053, and the like. In theportable information terminal illustrated in FIG. 18F, a modem may beincorporated in the housing 7051. The signal processing circuitaccording to one embodiment of the present invention can be used for anintegrated circuit used for controlling driving of the portableinformation terminal. With use of the signal processing circuitaccording to one embodiment of the present invention for the integratedcircuit for controlling driving of the portable information terminal, aportable information terminal can reduce power consumption.

This example can be combined as appropriate with any of theabove-described embodiments and examples.

This application is based on Japanese Patent Application serial no.2010-189214 and filed with Japan Patent Office on Aug. 26, 2010 andJapanese Patent Application serial no. 2011-113178 filed with JapanPatent Office on May 20, 2011, the entire contents of which are herebyincorporated by reference.

What is claimed is:
 1. A semiconductor device comprising a storageelement comprising: a first inverter; a second inverter; a firstselection transistor; a second selection transistor; a first transistorcomprising an oxide semiconductor layer; and a second transistorcomprising an oxide semiconductor layer, wherein an input terminal ofthe first inverter is electrically connected to an input terminal of thestorage element via the first selection transistor and the secondtransistor, wherein an output terminal of the first inverter iselectrically connected to an output terminal of the storage element viathe second selection transistor, wherein the output terminal of thefirst inverter is electrically connected to an input terminal of thesecond inverter via the first transistor, and wherein an output terminalof the second inverter is electrically connected to the input terminalof the first inverter via the second transistor.
 2. The semiconductordevice according to claim 1, wherein the first inverter and the secondinverter each includes a pair of transistors.
 3. The semiconductordevice according to claim 1, wherein the oxide semiconductor layercontains at least indium or zinc.
 4. The semiconductor device accordingto claim 1, wherein an off current per micrometer in a channel width ofthe first transistor and the second transistor is less than or equal to1×10⁻²¹ A/μm.
 5. A semiconductor device comprising a storage elementcomprising: a first inverter; a second inverter; a first selectiontransistor; a second selection transistor; a first transistor comprisingan oxide semiconductor layer; a second transistor comprising an oxidesemiconductor layer; a first capacitor; and a second capacitor, whereinan input terminal of the first inverter is electrically connected to aninput terminal of the storage element via the first selection transistorand the second transistor, wherein an output terminal of the firstinverter is electrically connected to an output terminal of the storageelement via the second selection transistor, wherein the output terminalof the first inverter is electrically connected to an input terminal ofthe second inverter via the first transistor, wherein an output terminalof the second inverter is electrically connected to the input terminalof the first inverter via the second transistor, wherein an electrode ofthe first capacitor is electrically connected to the input terminal ofthe second inverter, and wherein an electrode of the second capacitor iselectrically connected to the input terminal of the first inverter. 6.The semiconductor device according to claim 5, wherein the firstinverter and the second inverter each includes a pair of transistors. 7.The semiconductor device according to claim 5, wherein the oxidesemiconductor layer contains at least indium or zinc.
 8. Thesemiconductor device according to claim 5, wherein an off current permicrometer in a channel width of the first transistor and the secondtransistor is less than or equal to 1×10⁻²¹ A/μm.
 9. A method fordriving a semiconductor device comprising a storage element, wherein thestorage element comprises a first inverter, a second inverter, a firstselection transistor, a second selection transistor, a first transistorcomprising an oxide semiconductor layer, and a second transistorcomprising an oxide semiconductor layer, wherein an input terminal ofthe first inverter is electrically connected to an input terminal of thestorage element via the first selection transistor and the secondtransistor, wherein an output terminal of the first inverter iselectrically connected to an output terminal of the storage element viathe second selection transistor, wherein the output terminal of thefirst inverter is electrically connected to an input terminal of thesecond inverter via the first transistor, and wherein an output terminalof the second inverter is electrically connected to the input terminalof the first inverter via the second transistor, the method comprisingthe steps of: supplying a potential of data to the input terminal of thefirst inverter via the first selection transistor and the secondtransistor; outputting a potential of the output terminal of the firstinverter as an output signal via the second selection transistor;supplying the potential of the output terminal of the first inverter tothe input terminal of the second inverter via the first transistor;supplying a potential of an output terminal of the second inverter tothe input terminal of the first inverter via the second transistor; andturning off the first transistor and the second transistor beforestopping the supply of a power-supply voltage to the first inverter andthe second inverter.
 10. The method for driving the semiconductor deviceaccording to claim 9, wherein the first transistor and the secondtransistor are controlled so that when one of the first transistor andthe second transistor is in an on state, the other is in an on state.11. The method for driving the semiconductor device according to claim9, wherein the first transistor and the second transistor have the sameconductivity type, and wherein a first control signal input to a gate ofthe first transistor and a second control signal input to a gate of thesecond transistor are the same signal.
 12. The method for driving thesemiconductor device according to claim 9, wherein the first inverterand the second inverter each includes a pair of transistors.
 13. Themethod for driving the semiconductor device according to claim 9,wherein the oxide semiconductor layer contains at least indium or zinc.14. The method for driving the semiconductor device according to claim9, wherein an off current per micrometer in a channel width of the firsttransistor and the second transistor is less than or equal to 1×10⁻²¹A/μm.